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The physiological side of the game is not lost sight the several acts and bye-laws that regulate or hamper, of, and is dealt with by the author in three short as the case may be, the particular work he has in chapters, while Part iv. is devoted to “lawn tennis hand. The author, judging by this, and by the titles

of his previous works, seems to be a good Samaritan encyclopædia," containing much miscellaneous in

and to take pleasure in pointing out the numerous formation useful to players, including a bibliography pitfalls that must be avoided by the man who would, of the literature on the subject, which, by the way, if possible, live at peace. The present book is is very considerable.

largely filled with a recitation of laws and of district The volume concludes with an account of the council requirements which no one would wish to history and growth of the game of lacrosse, by tains examples of work in very full detail and with

read unless under compulsion. The latter part conWilliam Harvey Maddren.

illustrations. The very complete index adds considerably to the

However, the author has not, as might have been utility of this publication, which should form expected, lost all interest in the progress of his subject welcome addition to any sportsman's library. in wrestling with these dismal details. For instance, In the second of these volumes, which is the com

on p. 2 he says :

“The author is one of a few surveyors who believe bined work of Messrs. G. Beldam and P. A. Vaile,

that all wide carriageways (where traffic is considerwe have another valuable contribution

to the

able), should have the channel in the centre instead literature of lawn tennis. Mr. Beldam presents us of at the sides, thus obviating the tendency of with 229 of his action photographs, all of which are vehicles to slide down the haunches of the road here beautifully reproduced. In his book on “Great towards the kerb. The gradient to the centre channel Golfers ” he showed how much could be learnt by from the kerb need not exceed 1 in 40."

Whatever advantages or the reverse there may be closely studying action-photographs, and in the present volume

in this plan, spectators on the pavement would no great lawn tennis players a

doubt prefer to this sliding in the direction similar attempt is rewarded with equal success in desired by the author, especially if the vehicles spite of the greater difficulties involved, since both happened to be quick motor-cars going in opposite player and ball are in rapid motion. The photo directions.

The author is to be complimented on performing a graphs here given are not casual snapshots, but taken specially to illustrate the positions occupied by tedious and uninteresting task for the general good.

C. V. B. players for particular strokes. Mr. Vaile, writes, so to speak, round these pictures, and in his breezy and

A Popular Guide to the Heavens. By Sir Robert S. straightforward style points out which in his estima- Ball, LL.D., F.R.S. Pp. xii +96; 83 plates tion are the good or bad points. This author is of (London: George Philip and Son, Ltd., *1905.) the opinion that the true science of the game is but

Price 155. net. dimly appreciated in this country, and it is his main This is a new edition of the “ Atlas of Astrol.nzy," endeavour throughout these pages to indicate in by the same author, which appeared in 1892, the which direction progress can be made. The lawn revision having extended even to the title of the book. tennis reader will find, therefore, much to think over

As before, star maps and pictures of the heavenly

bodies are the chief feature, but in many cases in these pages, and particular attention is drawn to

drawings have been replaced by admirable reproducthe first chapter, in which the racket, per se, and the tions of some of the finest celestial photographs at methods of holding it are discussed. Mr. E. G. present available. The star charts, comprising Meers contributes an interesting chapter on Ad- twelve maps indicating the aspect of the heavens vanced Tactics of the Single Game," while “ The

in the different months, and twenty others showing

much greater detail, are excellent in every respect, Half-Volley” is treated by Mr. G. A. Caridia.

and will meet the needs of those making a first acquaintance with the stars as well as of those who

may wish to observe interesting objects with teleOUR BOOK SHELF.

scopes of moderate aperture. A valuable feature in New Streets: Laying Out and Making Up. By

connection with the maps is an index to the planets, A. Tayler Allen. Pp. 175. (London : The Sanitary

whereby the positions of these bodies in each month Publishing Company, Ltd.) 35. net.

during the next fifty years may be approximately

ascertained. A very complete guide to observations This is not the sort of book that anyone but a proof- of the moon is also provided by the maps and catareader could read straight through, not even

logues of lunar formations. So far, the book reviewer or a surveyor or architect, for whom justifies its title, but the remaining parts give the especially it is written. This statement is not made impression of a scrap-book with pages still remaining by way of disparagement, quite the reverse, and the

to be filled, and pages which would have been filled author would be the first to agree to it.

differently by different owners.

The sun,

for In these days, with a multiplicity of petty and of example, is inadequately represented; the only photolocal bye-laws and regulations, all put together graph of a sun-spot which is given conveys no indicaprimarily and ostensibly to prevent scamping of tion of the dimensions of the spot, and there are no different kinds, but often, and the more so the more illustrations of faculæ or photographs in petty the authority, used as weapons to compel public chromatic light. A more serious omission, in a book spirited parties to go to unnecessary and extravagant which is styled a “guide," is the absence of all referexpense so that the members of the petty body or ence to the modes of observing the sun, although their friends may be the more prosperous, it is above careful drawings of the paths of spots at different all essential that the surveyor or architect or engineer times of the year are included. Again, there is an or even private individual, who has occasion to make elaborate chart of the planet Mars, but nothing to a new street or a cottage or a side-walk or a retro-show what the planet looks like in an ordinary spective drain should act warily, and have before him telescope.

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The text amounts to little more than a description Toning Bromide Prints. “ Photography " Bookshelf of the plates and is too scrappy to give a connected | Series, No. 16. By R. E. Blake Smith. Pp. view of the subject. The book, however, is well XV + 104. (London : Iliffe and Sons, Ltd., 1902) produced, and will be valued for its excellent star Price is. net. inaps and examples of celestial portraiture.

INSTEAD of producing a black and white bromide

print it is often desirable to change the normal tone Denkmäler mittelalterlicher Meteorologie. No. 1 to suit the subject photographed. There are many

(Schlussheft). Neudrucke von Schriften und Karten methods by which this change of tints can be obüber Meteorologie und Erdmagnetismus heraus- tained, and these pages are devoted to describing the gegeben von Prof. Dr. G. Hellmann. Pp. lviii + various processes that are available. The material on 269. (Berlin : Asher and Co., 1904.)

which this book is based first appeared in a series of

articles in Photography, but in the present handy This is the final volume of a valuable series of publi

form it will be found more convenient for workers. cations which we owe to the energy of Prof. Hell

The author gives a good detailed account of each mann. In them we have had brought before us the

case, and discusses the probable effect of the different more interesting abstracts and reprints of early works

processes on the permanence of the finished picture. dealing with meteorology and terrestrial magnetism.

Workers with bromide papers will find this book of Prof. Hellmann has thus made available to those

considerable service. interested in these subjects, the records of ancient times, which to many would have remained unread and possibly unknown.

LETTERS TO THE EDITOR. In the present volume, which deals more especially with meteorology, we have presented to us a set of

[The Editor does not hold himself responsible for opinions twenty-six separate parts ranging from the seventh

expressed by his correspondents. Neither can he undertake

to return, or to correspond with the writers of. rejected to the fourteenth century. Many others have been

manuscripts intended for this or any other part of NATURE. taken from printed works, but some of them, as we

No notice is taken of anonymous communications. ] are told in the preface, are here published for the first time.

Charge on the a Particles of Polonium and Radium, Further, many of these old texts have here been

With reference to the interesting letter on this subject by translated into German so that those who are not

| Prof. Rutherford in last week's NATURE, I should like to familiar with old Saxon, old English, old Norwegian, point out that in my paper “ On the positive electrification or Arabic will still be able to gain a good insight of a rays and the emission of slowly moving kathode rays into the ideas of the Middle Ages.

by radio-active substances (Proc, Camb. Phil. Soc., xiii., In the introduction to this volume Prof. Hellmann p. 49) I have described experiments which demonstrale gives a brief sketch of the character of meteorology | the communication of a positive charge of electricity to at these periods, and adds a short and interesting bodies struck by a rays from polonium or radium. ! summary of biographical facts relating to the writers

had considerable difficulty in disentangling this positive of the texts to which reference is here made. An

charge from the copious streams of slowly moving

negatively electrified corpuscles which appendix contains additions and corrections to the

I found were

given out by these substances, and the experiments earlier numbers.

in which I finally succeeded in doing this were not For the labour involved in bringing together and

completed until a few days after the reading of the paper on preparing this collection of old texts a large debt of

November 14, and are not referred to in the abstract quoted gratitude is due to Prof. Hellmann, and it is hoped by Prof. Rutherford. A description of them will be found in that from time to time, when further ancient writings the paper which has lately been published. I may take this are brought to light, he will render them in like opportunity of saying that I have recently found that manner so conveniently available.

uranium also gives out slowly moving corpuscles, so that

this effect seems a general property of radio-active subThe Birds of Calcutta. By F. Finn. Second edition. stances. The velocity of these corpuscles is very small comPp. vi + 136. (Calcutta : Thacker, Spink, and Co;

pared with that of the B rays, and is more nearly of the London : Thacker and Co., 1904.)

order of the velocity of the corpuscles emitted by metals when exposed to light.

J. J. THOMSON. The fact of a work reaching a second edition may Cavendish Laboratory, Cambridge, March 4. generally be taken as an indication that it has received the seal of public approval, and that it accord

A CONVERSATION I had with Prof. Bragg, of the Adelaide ingly needs no commendation from us. In the present

University, in passing through Adelaide last summer instance, a ready reception would seem to be assured

suggested some thoughts in regard to the nature of the

a rays which may be of interest in view of Prof. Rutherford's to the new edition, since many additions and im

letter in last week's NATURE. Prof. Rutherford announces provements have been made. The most important

that he has at last succeeded in detecting the positive charge addition is undoubtedly the series of life-like cuts

carried by the a rays of radium by using a magnetic firld of Indian birds, which adds very largely. to the to deflect and remove the slow-moving electrons present with interest of the little volume; but it is also satisfactory the a particles. He says, “I think these experiments unto find that the arrangement and nomenclature have doubtedly show that the a particles do carry a positiv been revised so as to bring the work into harmony charge, and that the previous failures to detect this charge with the volumes on birds in the “Fauna of British were due to the masking action of the large number of India," to which it may serve in some degree as an slow-moving electrons emitted from the plates." "These introduction. Mr. Finn has a vivacious, if some results, while they afford a welcome confirmation of the contimes flippant, style, which removes his works from

clusions drawn from the evidence of the magnetic and electric the “dry-as-dust” category; but in some cases,

deviation suffered by the a rays, do not, to my mind, finally

settle the question. as in the application of the term “ disreputable" to

It must be admitted that the a particles in ordinary cirthe babbler, we venture to think some of his epithets

| cumstances do carry a positive charge. Certain evidence, might be better selected. To a former resident the

however, seems to point to the conclusion that the a particle omission of the adjutant stork from the list of Calcutta

at the moment of its expulsion from the parent atom is birds seems strange, but it appears that for many

uncharged, and that it derives its positive charge from years these weird birds have ceased to visit the city

secondary causes, independently of, and subsequent to the of palaces.

R. L. expulsion process. To devise a crucial experiment which

would decide between the two views would be far from easy, and s=H,/H, is the ratio of magnetic forces in the electrobut as I interpret Prof. Rutherford's letter, the results there magnetic case. Now (2) asserts that the reflected wave gets given do not definitely disprove the view that the a particle smaller as the mirror goes faster, and vanishes when u=fv. is initially uncharged.

Or if the mirror be pushed against the radiation, the reI recently directed attention (** Radio-activity," p. 181) | flected wave gets stronger, and the resisting force stronger to the importance of the fact that in certain well-established until u=-4v, when it is infinite. The mirror could not be cases there appeared to be a simultaneous production of two pushed against the radiation faster than šv. positive charges in the disintegration of an electrically An immediate objection is that when u has risen to fv, if neutral atom. Thus in the disintegration of the emanation it be maintained at that speed it acts like a perfect absorber atom a positively charged a particle is expelled, and the to the incident energy. Moreover, since there is the pressure residue of the atom-the matter causing the excited activity Po left, why should it not accelerate the mirror ? 'But, if -is also positively charged, and is concentrated on the it does, p. becomes negative, and s becomes imaginary. negative electrode in an electric field. In a recent paper by Considered mechanically only, say by F=, the motion Bragg (Phil. Mag., December, 1904, p. 721), the following of m is quite determinate when u> tv, up to v, in fact. sentence occurs :-" It is easy to see that even if the a particle But electromagnetically it means that the energy in the is uncharged when it leaves the parent body, it must imme- reflected wave is negative. Now although there is nothing diately become positive, since in traversing an atom it is to object to quantitatively in a continuous transition from a just as likely to lose one of its own electrons as to take one away from the atom traversed.". As I am unaware that combined with equal lateral pressure, to its negative, a

Maxwellian stress consisting of a tension along an axis this consequence has received the attention it deserves,

pressure along the axis with equal lateral tension, still the perhaps I may be allowed to direct attention to its bearing on

negativity of the energy in the reflected wave causes diffithe present question. There is a fundamental distinction between the ionisation of the atom of a gas molecule by becomes of the gravitational type. That is, like imaginary

culty. The stress for both the electric and magnetic energy radiant electrons or B particles, and radiant atoms or a

electrifications attract, and unlike repel, or matter is particles. For in the latter case, if the atom struck suffers imaginary electrification in this comparison. The moving ionisation, the radiant atom is just as likely to be ionised

forces and energies are real. But let a real charge and an in the process also. The ionisation of a neutral atom consists in the detachment from it of an electron which

unreal one co-exist, the energy density becomes imaginary.

That is out of all reason in a real universe. forms the negative ion, the atom thereby becoming positively charged and forming the negative ion. Hence the radiant a

We should, I think, regard (2) as a demonstration that particle, if uncharged initially, will become positively charged (1) is untrue, in that (p.+pa)u is not the activity of the on collision with the atoms of the gas or other obstacle

force on the mirror, although P.+Pmay be actually the in its path, and at the same time will lose an electron. The

pressure of the radiation. In fact, in the electromagnetic

case, slow-moving electrons present with the a particles,” which

the variation of p constitutes a force on the ether Rutherford describes as “ emitted from the plates," may

itself. We must find the force on the mirror in another way. therefore in reality be derived from the a particles them

Let radiation fall flush upon the plane surface of a dielectric, selves in the act of becoming positively charged. The fact

which call glass, moving the same way at constant speed u, that they, unless deflected by a magnetic field, exactly

and let the circuital equations in the glass be neutralise the charge carried by the a particles seems to

dH/dx = cE+01/01, -dE/dx=B=uH; (3) point in the same direction.

In further support of the view that the positive charges on that is, the same as for the ether, with the addition of the both the radiant particle and the residue of the atom after electric current of polarisation 81/āt. The reference space disintegration are derived by collision with the gas molecules, is the fixed ether, and alat is the moving time differentiator. Prof. Rutherford's results on the distribution of the excited Now if the relation between I and E is such as to permit of activity in an electric field at low pressure may be cited an undistorted plane wave, we shall have (Rutherford, “Radio-activity,” p. 282). If the excitedactivity-matter particle gains its positive charge in its recoil

E, = uvH, E,= - uvH2, Eg=uwHz, (4)

(incident) (reflected) (transmitted) by collision with the gas molecules, it is to be expected that at low pressures it will not become charged, and will not, if v is the speed in the ether, and w the wave speed referred therefore, be concentrated on the negative electrode, as is,

to the ether in the glass. This w is a function of u. Also, in fact, the case. FREDERICK Soppy. the boundary conditions, E+E, = E, H+H,=Hg,

(5) The Pressure of Radiation.

combined with (4), give The success of Lebedeff and Nichols and Hull in recog

H,/H, =(v W)/(v +w), H2/H,=2v/(v+w). (6) nising and measuring the pressure of radiation has aroused An incident pulse of unit depth is stretched to depth much interest in radiation pressure generally, real or ap- (1-u/v)-' in the act of reflection; the reflected pulse is of parent. It has some interesting and sometimes somewhat depth (v+u) (v – u)-', and the transmitted pulse of depth difficult theoretical aspects. In the first place, if the ether (W– u) (v- u)-1. is really absolutely at rest (this rigidity is a very difficult The rate of loss of energy from the waves in the process idea), the moving force on it has no activity, and its time of reflection is integral VDB can only be called momentum out of compli

Pilv u) Po(v + u) P3(w u),

(7) ment. The force becomes active in a moving ether, with where the p's are the energy densities. But, by the above, interesting consequences not now under examination. The present question is rather how to interpret the pressure of

Div=P2v+Pw;

(8) radiation on the assumption of a fixed ether, in the measure

therefore the rate of loss of energy is of its effects on matter which is either fixed or moving

(Ps - Pi-P2)u,

(9) through the ether. The following is striking in what it proves. Let plane

and the moving force on the mirror is radiation fall flush upon a perfect reflector moving in the

F=P3 - Di-P2.

(10) same direction at speed u, a case considered by Larmor. Let This is, in its expression, exactly the negative of the the energy density p=p,+pa, the incident' being Pi, the previous pressure difference. It is in the direction of the reflected pa. Assume, which seems reasonable at first, that rise of energy density. Its amount is Do the pressure in the reflector, is zero, then the moving F=2uHH,=2p1 (v - W)/(v +w)= {uH; -- AcE; = U. (11) force P+P:-p, reduces to Pi + P2. Therefore

The first form in terms of H,H, is useful. The second is Pilv u) – Po(v + u)=(1+Pa),

(1)

in terms of the wave speeds. The third is in terms of the because the left side is the rate of loss of energy from the ethereal energy inside the glass. All these come out of the waves, and the right side the activity of the force on the ratios H1, H. &c. Now the electric energy equals the reflector. So

magnetic energy in the transmitted wave. Consequently (2)

1, means the energy of the polarisation I. And the activity Pi 1+ 2u/v

is l',!, the convective flux of energy.

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These properties are true for various relations between This discovery has proved useful in the investigation of I and E. The first approximation is I =c, E. The second, secondary radiation proceeding from solids. introduced by Lorentz, is I =c,(E – uB), that is, the polarisa- It has been found that while the intensity of secondary tion is proportional to the moving force on a moving ion. radiation from light substances varies considerably in Other forms allowing of undistorted pulse propagation may different directions owing to the partial polarisation of the be proposed. All give special relations between w and u. primary radiation, the amount of this variation diminishes In Lorentz's case,

with an increase in the atomic weight of the radiator, and 0,= $4, E3(1 - ulw)?

(12) ultimately is inappreciable. The radiations from air, carbon,

paper, aluminium, and sulphur vary in intensity in different To pass to perfect reflection, reduce w to u, its least value.

directions by a considerable amount. From calcium the U. does not vanish, but has the value given by (10), (11) variation is much less, while from iron, copper, zinc, and still, with w=u. But the transmitted wave is reduced to a

lead it is inappreciable. This must be connected with the surface film, moving with the glass. The moving force on

fact that the radiation from light substances differs in the glass is now

character only very slightly from the primary, while the F=2P, (w u)/(v + 11),

(13)

heavier substances emit radiations differing more from the and finally, if uro, F=2pı:

primary producing them. The radiation from the heavier Here we come right back to the pressure of radiation.

metals was found not to consist of an easily absorbed radiaIt does measure the force on the glass when at rest, when

tion superposed on a radiation such as proceeds írom light it reflects perfectly, and it looks as if (13) were merely the

substances, and of intensity given by the law found for that form P. + P2 a little modified by the motion. But appear

from light substances, but is as a completely transformed ances are very deceitful here, for (10) above is the proper

radiation. This is strong evidence that the freedom of formula.

motion of the electrons which permits what may be called As regards the distribution of F. With an actual trans

a simple scattering in substances of lower atomic weight is mitted wave consisting of a pulse of uniform intensity all

interfered with in the heavier atoms, for we find from them through, F is entirely at the wave front. So, with total re

a more absorbable radiation in place of, not simply superflection, it is just under the surface of the glass. Again, if posed on, a more purely scattered radiation. E, varies continuously in the transmitted wave, F is dis

With this change in character, the polarisation effect distributed continuously, to the amount B(1/21) per unit volume.

appears. No special absorption of the radiation proceeding What F means in (11) now is the total of this volume force,

from a substance by plates of the same substance has been i.e. the integral from the surface up to the wave front, ex

observed. pressed in terms of the momentary surface state.

A considerable variation in the penetrating power oi the After a pulse has left the surface there is an equal opposite

primary radiation incident on heavy substances is accomforce at its back, so there is no further loss of energy or

panied by a smaller change in that of the secondary moving force on the glass. The obscurities and apparent

(measured by change of absorbability). contradictions arise from the assumption that the ether is

Radiation from compounds appears to be merely a mixture quite motionless. If we treat the matter more compre

of the radiations which proceed from the separate elements hensively, and seek the forces in a moving ether, with

in the compound, both the absorbability and polarisation moving polarisable matter in it as well, if this is a com

effects being what would be given by such mixtures. plication one way it is a simplification in another, viz. in

Atomic weight, not molecular weight or density, thus seems the ideas concerned. There is harmony produced with the

to govern the character of the radiation produced by a given stress theory. To illustrate, (a/at)VDB is the moving force

primary. per unit volume when the ether and polarised matter have a

These results may be accounted for by considering the common motion, D and B being the complete displacement atoms the electrons are far enough apart, and have sufficient

electrons constituting the atoms as the radiators. In light and induction. (The variation of u is ignored here.) But if we stop the ether, a part of this force becomes inactive.

freedom to move almost entirely independently of one another, If the matter is unmagnetisable, the only active part is that

under the influence of the primary pulses, consequently to containing the polarisation current, for that is carried along.

emit a secondary radiation similar to the primary, but the Besides this electromagnetic force, there is also a force with regard to that of electric displacement in the primary

intensity of which depends on the direction of propagation due to a pressure of amount U.. But it does not alter

beam. In heavier atoms considerable inter-electronic foros the reckoning of the moving force on the glass, because

are probably brought into play by small displacements, and the pressure acts equally and oppositely at the front and the resultant acceleration of motion of an electron is then back of a pulse.

not in the direction of electric displacement of the primary Some other illustrations of the curious action between electromagnetic radiation and matter can be given. For Also there ceases to be a simple connection between the

beam, and evidence of polarisation of that beam vanishes example, two oppositely moving plane pulses inside moving time for which the electron is accelerated and that of passage glass. Say E, = uw,H, one way with the glass, and

of the primary pulse. E,= - uw,H, against the glass. If H,=-H,, work is done

In atoms of greater weight we would expect appreciable upon the glass when they cross, ceasing the moment they inter-electronic forces to be called into play sooner, and to coincide, so that the energy of the momentary electric field attain a much greater intensity than in lighter atoms. is less than the wave-energy. On separating, the loss is The precise connection between the atomic weight of the restored. If, on the other hand, E, = - E,, work is done by radiator and the absorbability of the radiation is being the glass on the waves when uniting, so that the momentary investigated.

CHARLES G. BARKLA magnetic energy, together with the polarisation energy, is greater than the wave energy. In this second case, too, it

L'niversity of Liverpool, March 1. is noteworthy that the solitary waves are of unequal energy, whereas they are equal in the first case. But details must be omitted, as this communication is perhaps already too

Dates of Publication of Scientific Books. long.

OLIVER HEAVISIDE.

I HAVE just bought a copy of “A Treatise on Slate and February 21.

Slate Quarrying, Scientific, Practical, and Commercial," by
D. c. Davies, F.G.S., fourth edition, dated 1899 (Crosty

Lockwood and Son).
Secondary Röntgen Radiation.

To my astonishment, I find no statistics of later date in In a paper read before the Royal Society on February 16. it than 1876, e.g. p. 33, statistics of 1872 and 1873, p. 58 I described experiments demonstrating the partial polarisa- list of quarries in 1873, p. 59, production in 1876, p. 04. pro tion of Röntgen radiation proceeding from an X-ray bulb, duction last year (1876), p. 170, prices of slates in Londos and at the same time verifying the theory previously given last year (1876). of the emission of secondary X-rays from gases and light As the Home Office publishes annually a general report solids subject to Röntgen radiation.

and statistics of mines and quarries, and also a list of mion Later experiments have shown that beams of X-radiation and quarries, there is no excuse for the book being so cut may be produced exhibiting a greater amount of polarisa- of date in its statistics.

B. Honsos. tion than there was evidence of in the original experiments. The Owens College, Manchester, February 21.

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