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FIG. I.

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Locusts in the Red Sea.

To prove that the equivalent of PQ, QR is PR. A GREAT Right of locusts passed over the s.s. Golconda on (1) The equivalent of two perpendicular lengths is equal in Sovember 25, 1889, when she was off the Great Hanish Islands length to their hypothenuse. be the Red Sea, in lat. 13°:56 N., and long. 42° 30 E.

For, draw AD perpendicular to hypothenuse BC. The particulars of the flight may be worthy of record. It was first seen crossing the sun's disk at about it a.m. as a tense white flocculent mass, travelling towards the north-east at Row the rate of twelve miles an hour. It was observed at noon by the officer on watch as passing the sun in the same state of een sity and with equal speed, and so continued till after 2 p.m.

The light took place at so high an altitude that it was only visible when the locusts were between the eye of the observer and the sun ; but the flight must have continued a long time after : 2 p. m., as numerous stragglers fell on board the ship as late as

B

D The course of Might was across the bow of the ship, which at li the time was directed about 17° west of north, and the Aight

*** evidently directed from the African to the Arabian shore of - Ked Sea.

Then, let BD, DA = k . BA, making angle with BA The steamship was travelling at the rate of thirteen miles an

towards BD. brur, and, supposing the host of insects to have taken only four

Then, by similar triangles, AD, DC = k . AC, making angle

with AC towards AD. bours in passing, it must have been about 2000 square miles in tent.

But these equivalents are at right angles, and proportional to Sone of us on board amused ourselves with the calculation

BA and AC. Hence, their equivalent, by similar triangles, is Hu, if the length and breadth of the swarm were forty-eight k. BC along BC. widos, its thickness half a mile, its density 144 locusts to a cubic

But BD, DA, AD, DC = BC. .. k = 1;., k = 1. koi, and the weight of each locust of an ounce, then it

(2) If theorem holds for right-angled triangle containing would have covered an area of 2304 square miles; the number angle o, it holds for right-angled triangle containing 10. of insects would have been 24,420 billions; the weight of

For, let ACD = 0, where D is 90°. Produce DC to B, such

that CB = Le tous 42,580 millions of ions; and our good ship of 6000

CA. Then ABD

- 10. uns burden would bave had to make 7,000,000 voyages to carry

A this grea: host of locusts, even if packed together in times more cieseis than they were flying.

W: J. Wilson, the chief officer of the Golconda, permits me to say that he quite agrees with me in the statement of the facts given above. He also states that on the following morning another fight was seen going in the same north-easterly direction from 4132 m. to 5 a.m. It was apparently a stronger brood and more closely packed, and appeared like a heavy black cloud

0 vaibe herinn.

B

с The beasts were of a red colour, were about 24 inches long, and ,' of an ounce in weight. G. T. CARRUTHERS.

FIG. 2.

Then assume CD, DA = CA. Add BC. .: BD, DA = BC, CA.

But BD, DA = BA in magnitude by (1); and BC, CA has its equivalent along BA, ::: BC = CA. BD, DA = BA, both in magnitude and direction.

(3) If the theorem holds for @ and p, it holds for 0 + 0. For make the well-known projection construction. Thus

A Marine Millipede. It may interest “D. W. T.” (NATURE, December 5, p. 104) iu kow that Geophilus maritimus is found under stones and sms weets on the shore at or near Plymouth, and recorded in my “Faana of Devon," Section “Myriopoda," &c., 1874, published in the Transactions of the Devonshire Association for the Idvancement of Literature, Science, and Art, 1874. This speuze was not known to Mr. Newport when his monograph * 25 written (Linn. Trans., vol. xix., 1845). Dr. Leach has aven a very good figure of this species in the Zoological Way, vol. iii. pl. 140, Figs. I and 2, and says:

“ Habitat in birlannia inter scopulos ad littora maris vulgatissime.” But,

far as ny ob ervations go, I should say it is a rare species. Set Zoologist, 1866, p. °7 for further observations on this animal.

EDWARD PARFITT. Laeter, December 9, 1889.

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FIG. 3.

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Proof of the Parallelogram of Forces.
The objection to Duchayla's proof of the "parallelogram of

M
forces” is
, I suppose, admitted by all mathematicians. To

N hase the fundamental principle of the equilibrium of a particle on the "transmissibility of force," and thus to introduce the onception of a rigid boiy, is certainly the reverse of logical pro- OP = OQ, QP = ON, NQ, QR, RP OM, MP. criture. The substitute for this proof which finds most favour with modern writers is, of course, that depending on the (4) Finally, by (I), theorem holds for isosceles right-angled * parallelogram of accelerations.” But this is open to almost triangle ; by (2) it holds for right-angled triangle containing 5 serious objections as the other. For it introduces kinetic angle 90° = 2"; .. by (3) it holds for right-angled triangle conHleas which are really nowhere again used in statics. I taining angle m. 90o = 2" : i.e. for any angle (as may be shown, hold therefore propose the following proof, which depends on if considered necessary, by the method for incommensurables in sety elementary geometrical propositions. The general order of Duchayla's proof). argument resembles that of Laplace.

Hence, if AD be perpendicular on BC in any triangle, I wopt the "triangular” instead of the “parallelogrammic ”

BA, AC BD, DA, AD, AC = BC. Q.E.D. frem. "Tbus, if PQ, QR represent in length and direction any directed magnitudes whatever, and, if these have a single equi.

W. E. JOHNSON. sent, that single equivalent will be represented by PR.

Llandaff House, Cambridge, November 12.

Datice."

1

Glories.

rately operate. The administration of the anæstheti MR. JAMES McConnel asks in NATURE (vol. xl. p. 594) something, then, outside the diseased condition; so t. for acccounts of the colours and angular dimensions of glories. its use ought theoretically to be perfectly harmless to I saw a good instance of the phenomenon on Lake Superior, sick person. Unfortunately it is not always 30. June 17, 1888, and, having had my attention called to the value deaths from chloroform are, although rare, by no me* of accurate descriptions in such cases by Mr, Henry Sharpe's unknown. The administrator of chloroform is there me; “Brocken Spectres," I examined it caresully.

a person of great responsibility : he has to watch carto The shadow of my head on the mist was surrounded by a the effect of the agent on the patient, to brilliant halo or glory, slaty-white around the head, followed by unfavourable change that occurs, and to adopt mea-i22) orange and red; then a circle of blue, green, and red, and the same colours repeated more faintly.

to counteract any bad effects which appear.

The diameter of the innermost and brightest circle of red, as measured on the knowledge of the mode in which chloroform cats graduated semicircle of a clinometer, was 45°. There was also

danger to the life of the patient is therefore of vast r. a very distinct, but nearly white, fog-bow outside, of 42° radius,

portance ; for, if the administrator knows the sigtes as measured in the same way.

A. P. COLEMAN.

danger, there is more likelihood of counteracting a tra Faraday Hall, Victoria University, Cobourg, Ontario.

result. These fatal results, which are among the sad.. that occur in medical practice, ought, if possible, to

avoided. Fossil Rhizocarps.

What, then, is the danger to life of chloroform? Or, REFERRING to Sir William Dawson's note on this subject in speak more fully, what particular part of the body dies NATURE of November (p. 10), we regret that we have been chloroform injuriously affect when there is danger. This unable to trace the original source from which the statement in just the point that the various Commissions have attemped our “Hand-book of Cryptogamic Botany was derived, relative to settle. In the Scotch schools, more especially tha: to the fructification of Protosalvinia or Sporangites. The sentence Edinburgh, it has been taught that the great danger will therefore, with apologies to Sir W. Dawson, be removed

chloroform was in failure of respiration ; ineaning by it's from future editions of the work.

ALFRED W. BENNETT.

that the danger-signal of chloroform was the stoppaye irregularity of the breathing. As a corollary to this beline

it was considered that the heart was only atiected alla. The Arc-Light.

the breathing had become interfered with ; that, in tr Would you or any of your readers kindly tell me where I the respiration stopping, the blood was not oxygenatt*. may find an account of any of the latest methods of determining so the heart stopped beating. This was the teaching .. the back E.M. F. of the arc-light? JOSEPH MCGRATH.

the great Edinburgh surgeon, Syme. The English ari Mount Sidney, Wellington Place, Dublin.

especially the London) teaching was almost disert opposed to this. It was taught, and is still taught in :

London schools, that the great danger from chloroform THE HYDERABAD CHLOROFORM arose from its effect on the heart, which stopped bezo COMMISSION.

before the respiration ceased. Which, then, of these iwi

doctrines is true, or are both true? THE HE appointment of a Commission at the present The decision of this question is, as we have stated, one

time to investigate the action of chloroform as an of vasi importance; but it must be remembered thes, anæsthetic might to many seem an anomaly. For the whichever is right, the administrator of anesthetics alu.23 use of chloroform as an anæsthetic was introduced over pays attention to both the beating of the heart and the forty years ago: it was in November, 1847, that Prof. regularity of the respiration. Surgeon-Major Lawrie, one Simpson, of Edinburgh, first brought this valuable agent of the prominent members of the Hyderabad Chlorofort) before the medical profession. Since that time, the use Commission, says that it is possible to avert all risk en of chloroform has enormously extended, especially in the heart by devoting the entire attention to the respiratina our country, and although there are other valuable agents during chloroform administration." Medical opino in of the same class-such as ether and nitrous-oxide gas- England, both of that of experts (professional anesthesia yet there is a universality of opinion that the employment and of the general profession, is distinctly opposed to this of chloroform has in many cases a special advantage. view; and the administrator who does not attend to the Considering the extensive use of the agent, and the pro- pulse, as well as to the breathing, is certainly negleching gress which has been made of late years in the study of one of the main paths by which Nature shows us what 13 the action of drugs in man, it certainly is surprising that going on inside the organism. the knowledge of the effect of chloroform on the different From the statement of Surgeon-Major Lawrie it parts and organs of the body is not complete. This is quoted, it will be seen that the Hyderabad Chlomket. not altogether from want of attention to the subject; Commission came to the conclusion that the danger im because, previous to the Hyderabad Commission, at least the administration arose, not from the heart, but from the two Commissions were appointed with the view of investi- respiration. This view was strongly combated in our cougating the action of chloroform and its occasional serious temporary, the Lancet. The importance of the questios effects. These Commissions were appointed by the Royal led ihe Nizam of Hyderabad to obtain the services of i Medical and Chirurgical Society of London, and by the scientific medical man from England to go out to indis British Medical Association, and they were composed and attempt to settle the question. Dr. Lauder Bruntur of men who, from their knowledge of experiment and F.R.S., consented to go ; and, well known as be is for 1,5 acquaintance with practical medicine, were competent to life-long devotion to the experimental investigation of the discuss the question. The two Commissions arrived at the action of remedies and their practical application, it was same conclusions as the distinguished French man of considered probable that his aid in the research woulu science, Claude Bernard, had published years before, and lead to interesting and important results. From :) these conclusions tallied with the teaching of the great somewhat scanty news of the results which have been London medical schools.

telegraphed to England, it seems likely that the inves:Chloroform and other anæsthetic agents have a peculiar gation now progressing at Hyderabad will tend to position : they are powerful drugs used, not for disease revolutionize existing views as to the action of chloroform itself, but for the purpose of allowing an operation to be Dr. Brunton's views as regards the dangers of chloro performed, preventing the pain which would otherwise form before he left England were clearly expressed in his be felt, and relaxing the contraction and spasms of the well-known“ Text-book of Pharmacology." In it hesar muscles, so that the surgeon can more readily and accu- that “the dangers resulting from the employment

M

m

kaloroformn are death by stoppage of respiration and with which they are attracted downwards, it is evident eath by stoppage of the heart;" he lays as much stress that the balls, MM, which are insignificant in com3 the effect on the heart as on the respiration, and he parison with the size of the earth, can only exert ureeds to affirm that too strong chloroform vapour an extremely feeble attraction. So small is this that it

ty very quickly paralyze the heart. This view is, in-can only be detected when the beam is entirely inclosed reit, similar to the one we have already mentioned as in a case to protect it from draughts; when, further, the uhr in the London schools of medicine. It is also whole apparatus is placed in a room into which no one til known that death may occur soon after chloroform must enter, because the heat of the body would warm the niks begun to be administered, from the heart being case unevenly, and so set up air currents which would ted. If the operation is begun too soon, fainting have far more influence than the whole attraction to be

m pain may supervene, and a fatal result occur: this measured ; and when, finally, the period of oscillation is u always been strongly insisted upon by Dr. Brunton. made very great, as, for instance, five to fifteen minutes. Surgeon-Major Lawrie says that in such cases it is not In order to realize how small must be the force that will La se chloroform that acts on the heart, but simply that only just produce an observable displacement of the ere is fatal syncope or fainting.

balls, mm, it is sufficient to remember that the force From the large number of experiments on animals which brings them back to their position of rest is the which Dr. Brunton has performed in India, in conjunc- same as the corresponding force in the case of a pendulum : on with the Hyderabad Commission and a medical which swings at the same rate. Now a pendulum that delegate of the Indian Government, it appears that the would swing backwards and forwards in five minutes would * changer from chloroform is asphyxia or an overdose ; " have to be about 20,000 metres long, so that in this case nere is none whatever from the heart direct. This state- a deflection of one millimetre would be produced by a Tient is a distinct reversal of the view generally held in force equal to 1/20,000,000 of the weight of the bob. In England. It means that chloroform causes a fatal result the case of a pendulum swinging backwards and forlv; affecting the respiration or by too much being taken wards once in fifteen minutes the corresponding force into the system and affecting the brain ; and that there would be nine times as small, or 1/180,000,000 of the ! 30 direct paralysis of the heart from the chloroform. / weight.

perfectiy impartial opinion cannot, however, be formed In spite of the very small value of the constant of from the scanty records of the investigation which have gravitation, Cavendish was able, by making the appafren as yet received in England. We must wait for ratus on this enormous scale, to obtain a couple which tulier details of the experiments before a final judgment can be passed. le as well

, however, to point out that the prevailing view * England has been founded, not only on experiments on the lower animals, but also on the extended clinical obvalon of two generations of medical men. Clinical ob ervation is not so accurate or so lucid as that of direct experiment, but it has its value, and one by no means to

M' * iespised in a case where it is so extensive, and directed to a subject of such great importance, not only to the medical profession, but to the general public, as the question of the administration of chloroform.

would produce a definite deflection against the torsion of his suspending wire.

These measures ON THE CAVEVDISH EXPERIMENT.

were repeated by Reich (Comptes

rendus, 1837, p. 697), and then by Baily (Phil. Mag., 1842, In the last number of the Proceedings of the Royal vol. xxi. p. 111), who did not in any important particular

Society vol. xlvi. p; 253), I have given an account of improve upon the apparatus of Cavendish, except in the the improvements that I have made in the apparatus of use of a mirror for observing the movements of the Lavendish for measuring the constant of gravitation. beam. As the principles and some of the details there set out Cornu and Baille (Comptes rendus, vol. lxxvi. p. 954, vol. apply very generally to other experiments where extremely lxxxvi. pp. 571, 699, 1001) have modified the apparatus taitute forces have to be measured, it is possible that an with satisfactory results. In the first place they have 2Dstract of this paper may be of sufficient interest to find reduced the dimensions of all the parts to about onea place in the columns of NATURE.

quarter of the original amount. Their beam, an aluminium In the original experiment of Cavendish (Phil. Trans., tube, is only } metre long, and it carries at its ends 19. p. 46), as is well known, a pair of small masses, masses of pound each, instead of about 2 pounds, as *1 * Fig. 1), carried at the two ends of a very long but used by Cavendish. This reduction of the dimensions bigat torsion rod, are attracted towards a pair of large to about one-quarter of those used previously is conHasies, M M1, thus deflecting the arm until ihe torsion of sidered by them to be one of the advantages of their the suspending wire gives rise to a moment equal to that apparatus, because, as they say, in apparatus geometri. due to the attraction. The large masses are then placed cally similar, if the period of oscillation is unchanged, in the other side of the small ones, as shown by the the sensibility is independent of the mass of the susdorted circles, and the new position of rest of the torsion pended balls, and is inversely as the linear dimensions. dan is determined. Half the angle between the two I do not quite follow this, because, as I shall show, if all positions of rest is the detection produced by the attract the dimensions are increased or diminished together, the meg masses. The actual force which must be applied to sensibility will be unchanged. If only the length of the Ine ball: to produce this deflection, can be directly beam is altered and the positions of the large attracting sitermined in dynamical units when the period of oscilla masses, so that they remain opposite to, and the same true and the dimensions and masses of the moving parts distance from, the ends of the beam, then the sensibility lite known. In the original experiment of Cavendish, is inversely as the length. This mistake-for mistake it the arin is 6 feel long, the little masses are balls of lead surely is—is repeated in Jamin's “ Cours de Physique, 2. sl.ches in diameter, and large ones are lead balls i foot tome iv. ed. iv. p. 18, where, moreover, it is emphasized in diameter. Since ihe attraction of the whole earth on by being printed in italics. the smaller balls only produces their weight, i.e. the force The other improvements introduced by Cornu and

O

Fig. 1.

Baille are the use of mercury for the attracting masses It might be urged against this argument that a diff which can be drawn from one pair of vessels to the other culty would arise in finding a torsion fibre that would by the observer without his coming near the apparatus, give to a very short beam, loaded with balls that it was the use of a metal case connected with the earth to prevent safely carry, a period as great as five or ten minutes, and electrical disturbances, and the electrical registration of until quartz fibres existed there would have been a dio the movements of the index on the scale, which they culty in using a beam much less than a foot long, b.: placed 560 centimetres from the mirror.

it is now possible to hang one only half an inch in: The great difficulty that has been met with has been and weighing from twenty to thirty grains by a fibre nie the perpetual shifting of the position of rest, due partly more than a foot in length, so as to have a period of tive to the imperfect elasticity or fatigue of the torsion wires, minutes. If the moment of inertia of the heaviest beam but chiefly, as Cavendish proved experimentally, to the a certain length that a fibre will safely carry is so small tha: enormous effects of air-currents set up by temperature the period is too rapid, then the defect can be remedies differences in the box, which, with large apparatus, it is by reducing the weight, for then a finer fibre can be used, impossible to prevent. In every case the power of ob- and since the torsion varies approximately as the squar serving was in excess of the constancy of the effect of the strength (not exactly, because fine fibres carr actually produced. The observations of Cornu are the heavier weights in proportion), the torsion will be reduce only ones which are comparable in accuracy with other in a higher ratio, and so by making the suspended part. physical measurements, and these, as far as the few light enough, any slowness that may be required may live figures given enable one to judge, show a very remarkable provided. agreement between values obtained for the same quantity Practically, it is not convenient to use fibres much from time to time.

less than one ten-thousandth of an inch in diameter, and Soon after I had made quartz fibres, and found their these have a torsion 10,000 times less than that of value for producing a very small and constant torsion, I ordinary spun glass. A fibre one five-thousandth of an thought that it might be possible to apply them to the inch in diameter will carry a little over thirty grains. Cavendish apparatus with advantage. Prof. Tyndall, in Since with such small apparatus as I am now using :: a letter to a neighbour, expressed the conviction that it is easy to provide attracting masses which are very large would be possible to make a much smaller apparatus in in proportion to the length of the beam, while with large which the torsion should be produced by a quartz fibre. apparatus comparatively small masses must be made u-e The result of an examination of the theory of the instru- of owing to the impossibility of dealing with balls of lead ment shows that very small apparatus ought practically of great size, it is clear that much greater deflections i20 to work, but that in many particulars there is an advantage be produced with small than with large apparatus. For in departing from the arrangement which has always instance, to get the same effect in the same time from an been employed, conclusions which experiment has fully instrument with a 6-foot beam that I get from one in confirmed.

which the beam is five-eighths of an inch long, and the As I have already stated, the sensibility of the appa- attracting balls are 2 inches in diameter, it would be ratus is, if the period of oscillation is always the same, necessary to provide and deal with a pair of balls each independent of its linear dimensions. Thus, if there are 25 feet in diameter, and weighing 730 tons instead of two instruments in which all the dimensions of one are n about if pound apiece. There is the further advantage times the corresponding dimensions of the other, the in small apparatus that if for any reason the greates! moment of inertia of the beam and its appendages will be possible effect is desired, attracting balls of gold woulu as n : 1, and, therefore, the torsion also must be as ná: 1. not be entirely unattainable, while such small masses as The attracting masses, both fixed and movable, will be as two piles of sovereigns could be used where qualitative n3 : 1, and their distance apart as n:1. Therefore, the effects only were to be shown. Owing to its strongly attraction will be as n6n2 or nt: 1, and this is acting on magnetic qualities, platinum is unsuited for experiments an arm n times as long in the large instrument as in the of this kind. small; therefore the moment will be as n’:1; that is, in By far the greatest advantage that is met with in small the same proportion as the torsion, and so the angle of apparatus is the perfect uniformity of temperature wh:) deflection is unchanged.

is easily obtained, whereas, with apparatus of large size. If, however, the length of the beam only is changed, this alone makes really accurate work next to impossible and the attracting masses are moved until they are The construction to which this inquiry has led me, 270 opposite to, and a fixed distance from, the ends of the which will be described later, is especially suitable 1 beam, then the moment of inertia will be altered in the maintaining a uniform temperature in that part of the ratio no: 1, while the corresponding moment will only instrument in which the beam and mirror are suspendet. change in the ratio of n:1; and thus there is an ad- With such small beams as I am now using it is much vantage in reducing the length of the beam until one of more convenient to replace the long thin box generally two things happens : either it is difficult to find a suffi- employed to protect the beam from disturbance by a ciently fine torsion thread that will safely carry the beam vertical tube of circular section, in which the beam with and produce the required period-and this, I believe, has its mirror can revolve freely. This has the further adup to the present time prevented the use of a beam less vantage that, if the beam is hung centrally, the attraction than } metre in length-or else, when the length becomes of the tube produces no effect, and the troublesome and nearly equal to the diameter of the attracting balls, they approximate calculations which have been necessary to then act with such an increasing effect on the opposite i find the effect of the box are no longer required. The suspended balls, so as to tend to deflect the beam in the attracting weights, which must be outside the tube, must opposite direction, that the balance of effect begins to be made to take alternately positions on the two sides of fall short of that which would be due to the reduced the beam, so as to deflect it first in one direction and length of the opposite ball did not interfere. Let this then in the other. For this purpose they are most shortening process be continued until the line joining the conveniently fastened to the inside of a larger inetal tule. centres of the masses M M makes an angle of 45° with the which can be made to revolve on an axis coincident with line mm ; then, without further moving the masses MM, the axis of the smaller tube. There are obviously wil a still greater degree of sensibility can be obtained, pro- planes, one containing and one at right angles to the vided the period remains unaltered, by reducing the beam, in which the centres of the attracting balls will fic length of the beam m m to half its amount, so that the when they produce no deflection. At some intermediate distance between the centres of MM is 2 1/2 times the position the deflection will be a maximum. Now, it is new length mm, at which point a maximum is reached. a matter of some importance to choose this maxmum position for the attracting masses, because, in showing the two sides of the apparatus at different levels. Each the experiment to an audience, the largest effect should large mass is at or near the same level as the neighbourbe obtained that the instrument is capable of producing ; | ing small one, but one pair is removed from the level of -hile in exact measures of the constant of gravitation this the other by about the diameter of the large masses position has the further advantage that the only measure- which in the apparatus figured below is nearly five times meat which there is any difficulty in making, viz. the as great as the distance in plan between the two small angle between the line joining the large masses and the masses. Ine joining the small, which may be called the azimuth of In order to realize more fully the effect of a variety of "he instrument, becomes of little consequence under these arrangements, I have, for my own satisfaction, calculated circumstances. In the ordinary arrangement the slightest the values of the deflecting forces in an instrument in uncertainty in this angle will produce a relatively large which the distance between the centres of the attracting uncertainty in the result. I have already stated that if balls is five times the length of the beam, for every azian angle of 45° is chosen, the distance between the centres muth and for differences of levels of 0, 1, 2, 3, 4, and 5 of the large balls should be 2 2 times the length of the times the length of the beam. beam, and the converse of course is true. As it would The result of the calculation is illustrated by a series pot be possible at this distance to employ attracting balls of curves in the original paper. The main result, howwith a diameter much more tban one and a half times ever, is this. the length of the beam, and as balls much larger than this In the particular case which I have chosen for the inare just as easily made and used, I have found by calcula- strument, i e. where the distance between the centres of tioo what are the best positions when the centres of the M M and the axis, and the difference of level between the attracting balls are any distance apart.

two sides are both five times the length of the beam, as If the effect on the nearer ball only is considered, then seen in plan, and where the diameter of the large masses it is easy to find the best position for any distance of the is 6:4 times the length of the beam, the angle of deflection attracting mass from the axis of motion. Let P (Fig. 2) becomes 1867 times as great as the corresponding angle be the centre of the attracting ball, N that of the nearer in the apparatus of Cavendish, provided that the large

masses are made of material of the same density in the two cases and the periods of oscillation are the same.

Having now found that with apparatus no bigger than an ordinary galvanometer it should be possible to make an instrument far more sensitive than the large apparatus in use heretofore, it is necessary to show that such a piece of apparatus will practically work, and that it is not liable to be disturbed by the causes which in large apparatus have been found to give so much trouble.

I have made two instruments, of which I shall only describe the second, as that is better than the first, both in design and in its behaviour.

The construction of this is made clear by Fig. 3. To
N
M

a brass base provided with levelling screws is fixed the
vertical brass tube t, which forms the chamber in which
the small masses a b are suspended by a quartz fibre
from a pin at the upper end. These little masses are
cylinders of pure lead 11'3 millimetres long and 3 milli-
metres in diameter, and the vertical distance between
their centres is 50-8 millimetres. They are held by light

brass arms to a very light taper tube of glass, so that their FIG. 2.

axes are 6.5 millimetres from the axis of motion. The

mirror m, which is 12*7 millimetres in diameter, plane, and altracted ball, o the axis of motion, c and a the distances of unusual accuracy, is fastened to the upper end of the of P and x from o, and r the distance from n of the glass tube by the smallest quantity of shellac varnish. foot of the perpendicular from P on ON produced. Then Both the mirror and the plate-glass window which the moment of N about o will be greatest when

covers an opening in the tube were examined, and after

wards fixed with the refracting edge of each horizontal, 1+ za! + c 2002 – a),

so that the slight but very evident want of parallelism

between their faces should not interfere with the definior what comes to the same thing when

tion of the divisions of the scale. The large masses M M

are two cylinders 1 of lead 50.8 millimetres in diameter, cos: 8+ * + a?

and of the same length. They are fastened by screws to 3.

the inside of a brass tube, the outline of which is dotted Now, as the size of the attracting masses Mm is in- base, so that it may be twisted without shake through any

in the figure, which rests on the turned shoulder of the creased, or, as is then necessarily the case, as the distance angle. Stops (not shown in the figure) are screwed to of their centres from the axis increases, their relative the base, so that the actual angle turned through shall be action on the small masses mm at the opposite ends of that which produces the maximum deflection. A brass the beam increases, and so but a small fraction of the advantage is obtained, which the large balls would give

lid made in two halves covers in the outer tube, and

serves to maintain a very perfect uniformity of temperaIf they acted only upon the small balls on their own side.

ture in the inner tube. Neither the masses MM, nor For instance, if the distance between the centres of MM is five times the length of the beam, the moment due to is 160 seconds.

the lid, touch the inner tube. The period of oscillation the attraction on the opposite small balls is nearly half

With this apparatus placed in an ordinary room with 43 great as that on the near balls, so that the actual sensibility is only a little more than half that which would 1 Cylinders were employed instead of spheres, because they are more be obtained if the cross action could be prevented.

easily made and held, and because spheres have no advantage except when

absolute calculations have to be made. Also the verticai distance a b was I have practically overcome this difficulty by arranging for convenience made only about four times the length ab in plan.

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