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Dr. T. H. MONTGOMERY, in the Proceedings of the are liable to suffer from the ravages of a parasitic fungus, American Philosophical Society for the last quarter of Colletotrichum gloeos porioides, which attacks the leaves, 1904, runs a tilt at the generally accepted view as to the causes spot or canker on the fruit, or brings about morphological superiority of the male sex in animals. abscission of the inflorescence. The fungus has been reAmong invertebrates, he urges, it is always the male which ported from various orange-growing countries, and on is of inferior size and development, while as regards verte- account of its partiality for limes, planters in the West brates, although the males have in many cases secured Indies will do well to consult the account by Mr. P. H. superiority in the matter of bodily size and secondary Rolfs which is published in the Bulletin, vol. iii., part ii., sexual characters, yet, as regards the generative organs
of the Department of Agriculture, Jamaica. (notably the suppression in certain instances of one ovary), the advantage, from the point of view of specialisation, varieties, and market requirements of well known
The publication of pamphlets dealing with the cultivation and development, is largely on the side of the female. While admitting that different morphologists might esti
commercial plant products, as instituted by the director mate the value of these characters differently, the author
of the Royal Botanic Gardens, Ceylon, is a practical and is inclined to give the greatest morphological value to the important phase in the development of economic botany. higher development of the reproductive organs.
In vol. i., Nos. 23 and 25, of the Circulars of the gardens,
Mr. H. Wright takes up the subjects of ground nuts and In discussing in the same issue the origin of the mark- castor oil plants. The best quality of ground nuts, and ings of organisms, the late Prof. Packard arrived at the these can be grown in Ceylon, are bought for eating, but conclusion that these are dependent on the physical rather the demand is limited ; on the other hand, the requirethan on the biological environment. The alleged instances of the nuts for oil-crushing, although the price is of “ Müllerian " mimicry he explained, for example, less remunerative, is practically unlimited, and the cake by convergence due to the action of similar physical and furnishes an excellent cattle food. In the castor seed trade climatic causes, since he regarded the attacks of birds as a it does not appear that Ceylon will become a formidable negligible factor. Again, the frequent instances of colour
rival to India. and pattern resemblance between different animals he attributed to pigmentation caused by exposure to
The Cerro de Pasco silver mines are the most remarklight and shade, due to the repetition of fundamental
able in Peru, having been worked since the year 1630. AL colours. “To claim that Müllerian mimicry,” he added,
the present day operations are chiefly confined to the re“is due to the attacks of birds, is to overlook the fact of
working of old slags and waste heaps. On March 21, the existence of stripes, bars, and spots on the wings of
1902, a Government Commission was appointed to make palæozoic insects which flourished before the appearance
a survey of these mines, and the report of the commission of birds, and even of modern types of lizards."
has now been published in the form of a Boletin issued
by the Peruvian Corps of Mining Engineers. Illustrations The Report on the third outbreak of plague at Sydney and descriptions of the smelting works are given, and it in 1903 by Dr. Ashburton Thompson is interesting is noted that the output in 1903 amounted to 7213 tons of showing how an epizootic of plague among the rats pre- matte containing 4071 tons of copper. It is curious that ceded the two cases of human plague. From July 15, 1902, these ancient silver mines should develop as copper mines to April 30, 1903, 31,075 rats were caught, of which 17,160 in depth. were examined and found to be free from plague. On May 12 a rat was found on certain premises which on
A note in NATURE for January 26 (p. 305) referred 19
Adelaide, in South Australia, and Coolgardie, Western examination proved to be infected with plague, and up to
Australia, as the places having the highest maximum August 15 14,671 rats and mice were caught, of which
temperatures recorded in the British Empire. Mr. W. E 111 rats and 50 mice were ascertained to be infected with
Cooke, Government astronomer of Western Australia, writes plague. From then until December, 1903, 13,389 rats and mice were captured of which none was infected. The two
to say that Marble Bar, in the north-west division of that human cases occurred on June 20 and July 4, i.e. during
State, is very much hotter than Coolgardie. The mom the period when the epizootic existed among the rodents.
of the daily maximum temperatures for January, 1905, was
109°.8, and the highest reading 120°5. He adds that a The February number of Indian Public Health (i., No. 7) Jacobabad, in India, the average daily maximum temperacontains several papers of interest, notably one criticising
ture is 11°6 in May, 112°.7 in June, and 107°8 in July the plague policy of the Indian Government, in which it
and at Duem, in the Egyptian Soudan, the mean maximus is concluded that the only way to grapple with the plague
for March, 1902, was 11494, and the absolute maximura problem is the formation of a properly organised and
127° 4. equipped permanent public health service for the country.
We have received from Mr. J. van Breda de Haan : In the course of a report on the characters and analyses copy of a valuable series of meteorological observatinas of sweet potatoes cultivated in Jamaica, Mr. H. H. made during the year 1901 at the State Botanical Gardens Cousins, writing in the West Indian Bulletin (vol. V., at Buitenzorg, Java. The observations are made with the No. 3), records the fact that the process of cooking in
view of explaining certain problems connected with regte creases the sugar content of sweet potatoes very consider- table physiology, and consequently special attention is giers ably. Further experiments are being undertaken to ascer- to air and underground temperature, humidity and attain the exact chemical nature of the change. A com- shine, and more particularly to the intensity of rainfal parison of tubers freshly dug with others that had been showers. Observations and monthly means are given kr stored for some weeks indicated that during storage there several hours of each day, in addition to daily means is also a development of sugars at the expense of other substances in the tubers.
THE Quarterly Journal of the Royal Meter slagi.»
Society for January last contains an interesting paper Various kinds of citrus fruits, including oranges, pome- the decrease of fog in London during recent sears. The loes, grapefruit, and more particularly lemons and limes, results are given for months and for seasons for each
the thirty-three years 1871 to 1903, based upon the observ- Mr. R. Monti on the new find." This water mite was ations for London (Brixton) published in the daily weather obtained in cold springs on the right bank of the Anza, report of the Meteorological Office. The mean annual near Ceppomorelli, and has been named Polyxo placophora. number of foggy days is 55, of which 45 occurred in the The same writer in another number of the same journal winter half of the year. Dividing the thirty-three years
discusses the horizontal migrations of lacustrine plankton, into three equal periods, the result is, for the first period, and finds in mountain lakes that, in addition to the known a mean of 55, for the second 69, for the third only 41. vertical movements, there are well-marked diurnal migraSince the year 1888 a steady and uninterrupted decrease tions of the small crustacea to different parts of the lake is shown in the mean annual number of fogs. Among the depending on sunshine and shade. principal agencies which may have conduced to this desir
In the March number of the American Journal of Science able result must be mentioned the efforts of the Coal Smoke Abatement Society and the London County Council,
Mr. Charles S. Hastings utilises some observations of the also the use of incandescent gas light and electricity ;
power of accommodation of the eye for light of different but, as pointed out by Captain A. Carpenter and Mr. C.
wave-lengths to make a complete determination of the Harding, the increase of wind in recent winters is prob- optical constants of the eye for all conditions of accom
modation and for all colours. The results are given in ably chiefly responsible for the decrease of fog. As we have remarked before, the geographical situation of
two tables, by the use of which all problems connected London is, from a purely meteorological point of view,
with the purely optical properties of the schematic eye eminently favourable to the development of fog, and the
may be solved. only permanent improvement we can hope for is an abate
In the course of an investigation of radio-active muds ment of its more injurious effects caused by the imperfect which is published by Prof. G. Vicentini in the Atti of consumption of coal and gas.
the Royal Venetian Institute (vol. Ixiv., ii., 535), the conWe have received a copy of part i. of the “ Katalog
nection existing between the ionisation produced by the
mud and the quantity of material used is experimentally der Bibliothek der Naturforschenden Gesellschaft in
ascertained. When the mud is spread uniformly over a Danzig," published at Danzig in 1904. Although the
definite area, the intensity of the radiation increases as list of books included is not completely representative, this
the thickness of the layer is increased, but a direct propublication, containing the sections mathematics and
portionality does not exist between them. After a certain astronomy, may be found useful to those desiring to refer
point, moreover, the radio-activity is not increased by to the works of certain authors on these two subjects. adding fresh material. Mr. H. S. Allen, in a paper read The range of subjects is a wide one, and the books are
before the Royal Philosophical Society of Glasgow on entered under the names of the authors.
January 25, also deals with radio-active water and mud,
the material in this case being derived from the springs Having occasion recently to devise a short-focus spectro- of Bath and Buxton. An interesting point which is graph, Prof. Wood, of the Johns Hopkins University, established incidentally is that the fluorescence excited in found it necessary to make a study of the distribution of
a sensitive plate by the radium rays plays only a very light (monochromatic) in the different orders of a typical
minor part in the production by these rays of a photograting. His method, a beautifully simple one, is de graphic effect. scribed and illustrated in No. 2, vol. xxi.,, of the Astrophysical Journal. The result showed that, in the typical An interesting investigation of the secondary radiation grating experimented with, half the reflected light was produced when the B and y rays of radium impinge on concentrated in one spectrum, and the grating re- metallic plates is published by Prof. J. A. McClelland in flected about 76 per cent. of the total incident light, this the Transactions of the Royal Dublin Society (vol. viii., means that about one-third of this total was found in the No. 14). It is shown that the secondary rays are not proone spectrum, which was one of the two first orders. It duced merely at the surface of the plate struck by the was also found that the ruling makes little or no difference primary rays, but that they come from all parts of a layer to the total reflecting power of the speculum. Two flint of considerable depth. Apparently the less penetrating prisms of 60° would give about the same average dis- B rays are more efficient in producing a secondary radipersion as that produced, and, according to Pickering's ation than the wy or highly penetrating rays. The nature table in Kayser's “ Handbuch,' they would transmit a of the secondary radiation depends largely on the character little more than twice the light reflected, in the first order of the metal employed ; the greater the atomic weight of of the grating used.
the latter the greater is the amount of the secondary radi
ation produced by it. Of all the substances experimented The Psychological Bulletin, ii., 2, contains reports of with, lead gives rise to the greatest effect, both as rethe proceedings of the thirteenth annual meeting of the gards the quantity of the secondary radiation and its peneAmerican Psychological Association and of the fourth trating power. The secondary radiation consists, apparannual meeting of the American Philosophical Association, ently entirely, of a species of B rays, that is, of negatively which were both held at Philadelphia on December 28–30. charged particles capable of deflection in a magnetic field. Abstracts of the papers are given. Invitations on behalf of Perhaps the most important feature of the paper lies in its Harvard University to hold the next annual meeting in directing attention to the necessity of considering secondary Cambridge, Mass., to signalise the opening of the Emer- radiations in all measurements of the absorptive power of son Hall of Philosophy were accepted by both associations, substances with regard to the rays produced by radioand it is proposed that the Western Philosophical Associ- active bodies. ation and the Southern Society for Philosophy and Psychology shall also meet at the same time and place.
We have received a copy of a memorandum on the con
struction and verification of a new copy of the imperial A COLOU'RED plate of a new species belonging to a new standard yard, by Mr. H. J. Chaney, superintendent of genus of Hydrachnidæ is given in the Rendiconti of the the Standards Department of the Board of Trade. Since Lombardy Institution, xxxviii., 3. in illustration of a note by the original standard yard of bronze was made some sixty
years ago, it has been found that bars which are ductors, these facts being in contra
MR. W. Woops Smyril will give a bar without the intervention of gold plugs or pins as in
Bible in the Light of Modern Science" at the older type. Instead of using the old solid 1-inch Tichborne Street, Edgware Road, to-moi, section, for the purpose of lightness the so-called
at 5 p.m. “ Tresca " section has been adopted. The memorandum gives full details of the verification of the length and a
Messrs. WATTS AND Co. will shortly publ description of the apparatus used, including the thermo
Rationalist Press Association, Prof. Haeckel's meters by which temperature was measured and a
of Man," being a translation of the recently microscopic comparator similar to that used at Paris
edition of Anthropogenie.” by the Comité international des Poids et Mesures. This instrument has been purchased by the Board of Trade and mounted in a special chamber at Old Palace Yard, West
OUR ASTRONOMICAL COLUM minster.
Comet 1905 a (GIACOBINI).-A second telegi
the Kiel Centralstelle announces that comet to Vessels of fused quartz can now be obtained com
observed by Prof. Aitken at Lick on March mercially, and on account of the remarkable properties of position at March 27d. 7h. 57.1m. (Lick MT this substance, a wide field of research at high temperatures R.A. = 5h. 48m. 55s., dec. = +12° 35' 43". would appear to be opened up by their use. In high
Apparently, then, the northern declination is inc temperature gas thermometry, for example, where glass is
and not decreasing as previously stated. An erros excluded on account of its comparatively low melting point, declination for N.P.D., so that the daily move
key by which the code telegrams are translated su and platinum on account of its permeability to hydrogen, declination should be read as plus 1° 15'. fused quartz promised to be an ideal envelope. Unfortunately, The following elements have been computed buyers Villard has found that fused quartz is also permeable to
Strömgren from observations made on March 20, hydrogen at high temperatures, well below its melting
30, and are given in Circular No. 76 of the Kiel point, and Jacquerod and Perrot have proved that helium
stelle, together with a bi-daily ephemeris extendin
March 30 to April 23 :resembles hydrogen in this respect. In the current number of the Comptes rendus (March 27) M. Berthelot shows that
Elements. the use of quartz vessels is still further limited, as both
T = 1905 April 3'2098 (M. T. Berlin). oxygen and nitrogen can penetrate into hermetically sealed
00 = 357° 9:49 quartz bulbs at 1300° C. Thus carbon, heated in sealed
38 = 156° 7'94 19050 vacuous quartz tubes for half an hour at 1300° C., gave
i = 41° 37':48 a mixture of nitrogen and carbon monoxide on cooling
log 9 = 0'05232 the tube and extracting the gases. Experiments were made on other substances, and all the facts pointed to the
Ephemeris 12h. (M.T. Berlin). conclusion that at a high temperature fused silica behaves
log towards gases like an animal membrane, susceptible of April 7
6 31 16 +25 26'9 9-8661 endosmosis and exosmosis, the phenomenon depending
6 40 5 + 27 39'9 partly on the thickness of the wall. It is clear, therefore,
6 49 13
+29 48:1 9.8745 093 that before this substance can be used with confidence in
15 high temperature work, a further study will have to be
+ 33 479 9:8855 ot; made of its defects in this direction.
Brightness on March 26 = 1.0. The Comptes rendus for March 27 contain an interest
PHOTOGRAPHY OF THE CORONA WITHOUT A TOTAL ECLIPSE
--According to note communicated ing paper on the cryoscopic behaviour of hydrocyanic Academy of Sciences, and in the opinion of M. J. Jansert
to the Fren acid, by M. Lespieau. According to the views of Nernst M. A. Hansky has succeeded in photographing the corona and Thomson on the relation between the dielectric capacity
of the uneclipsed sun. The photographs were taken with and the power of electrolytic dissociation, the fact that the
a 12-inch telescope in the exceptionally transparent atmodielectric constant of prussic acid is higher than that of sphere which obtains at the observatory situated
summit of Mont Blanc. water should give the acid a higher dissociating power. After a number of preliminary experiments on M. Lespieau has accordingly carried out a series of ex- selective absorption of screens dyed with various anilise periments on the lowering of the freezing point of this colours, M. Hansky obtained a combination which absorbent substance by the addition of alcohol, chloroform, benzene,
all radiations more refrangible than 660 Mult, and, as to water, trichloracetic acid, sulphuric acid, potassium iodide
red radiations of the corona are very intense and done
suffer absorption or dispersion in passing through and nitrate, and has found that for the first six substances terrestrial atmosphere, he used this screen in obtaints the cryoscopic constant is between 19 and 20, whilst for twelve negatives. The individual screens were prepan the two latter it is approximately double. Hence the two by soaking a fixed undeveloped Lumière film in each acids, which are strongly dissociated in water, are not
the suitable dyes, and, bei ween each exposure, they *** sensibly dissociated in prussic acid solutions of the same
re-arranged inter se so that no false effect due to an strength, and this is in accord with the experiments of resulting picture.
particular disposition of the "grain " might after
The direct photospheric and chrow Kahlenberg, who found that these solutions were bad con- spheric rays were prevented from reaching the place to
7 8 22
the interposition of a blackened brass disc slightly larger within the above limits. A further result obtained was than the solar disc.
that the analogous wave-lengths in the arc and the spark The resulting negatives showed distinct halos around spectra of the same elements are not measurably different. the disc, and, notwithstanding the fact that some time elapsed between the successive exposures, these halos ex
STATISTICS OF VARIATION.1 hibited the same form, thus testifying to their solar origin. Some of the negatives were photographically A PAPER consisting mainly of a large number of intensified repeated copying, and reproductions of them
elaborate records bearing on the important subject were submitted to the academy. In presenting the com
of variation has recently been issued by the Washington munication M. Janssen—to whom M. Hansky acknow- | Academy of Sciences. The data, which have been collected ledges his obligations for assistance and advice-stated with much care and industry, cannot fail to be of high that the photographs actually show the solar corona
interest to all students of evolution. They afford an with an intensity and a perfection only known on the excellent example of the peculiar value of insect studies in photographs obtained during total eclipses (Comptes reference to many difficult problems in biology-a point rendus, No. 12).
which has lately received fresh emphasis from Prof. SEARCH-EPHEMERIS FOR TEMPEL's First PERIODIC Comet
Poulton's valedictory address as President of the Entomo(1867 11).-Although Tempel's first comet has not been
logican Society of London, seen during its last three perihelion passages, i.e. since
The aut.crs start with an Introduction," in which 1879, M. A. Gautier, of the Geneva Observatory, thinks
they declare tncir “ belief in the marked betterment and that the probability of its re-discovery this year is great
effectiveness of practically all variation study when pursued enough to justify a careful search. For this reason he
from the point of view of the biometrician”; adding, howre-publishes, in No. 4008 of the Astronomische Nach
ever, that “ from the writers' point of view the study of richten, the elements he prepared for the 1898 apparition,
variation is a phase of biology, and not of mathematics." reduced to the mean equinox of 1905.0: As the probable data in this inquiry, they assert that the phenomena of
Dealing with the special advantages presented by insect time of perihelion is somewhat uncertain, he gives three ephemerides, extending from March 31 to July 13, in
complete metamorphosis afford a lady means of distinwhich this time is reckoned as May 2.5, April 20.5, and
guishing “ variations which are strictly blastogenic from April 8.5 respectively, the mean date being the most prob
others which may be in large part acquired." This, it able. The declination varies from – 16° to -31°, so that
may be remarked, is only true under certain limitations. the more southerly observatories are
It is not the case, for instance, as the authors appear to
more likely to be successful in the research.
think, that the imaginal colour-patterns of lepidoptera are
uninfluenced by the conditions obtaining during individual Right ASCENSIONS OF 2120 SOUTHERN STARS.-In an development. appendix to “ Observations made at the Hong Kong Coming now to the main substance of the paper, we Observatory during 1903,” Prof. W. Doberck, the director, find a series of short articles or sections giving statistics of publishes the right ascensions of 2120 southern stars for variation in some two dozen species of insects. Among the the epoch 1900, as determined from observations made by structures thus dealt with are the venation and costal Mr. j. I. Plummer and himself during the years 1898 to wing hooks in bees and ants, the venation in gnats, the 1904.
colour-patterns of sundry beetles, wasps and bugs, the The observations were made with a 3-inch Simms semi- eye-spots of certain butterflies, the tibial spines, tarsal and portable transit instrument, which, together with the antennal segments, tactile hairs and elytral striæ of other method of reduction and the comparisons with other cata- insects of various orders. In the case of the hive bee it logues, is briefly discussed in the director's preface.
is incidentally shown that the parthenogenetically proIn the catalogue itself, the number of the star as given duced drones are as subject to variation in their wings as in Lacaille, or Stone, or both, the R.A., epoch and magni- are the workers of biparental ancestry. The results are tude, the variation of the R.A. from Stone's correspond in many cases graphically summarised in the form of the ing value, the proper motion, and several other particulars frequency polygon; and the mode," standard deviaare given for each star.
tion,” "index of variability," and coefficient of variaTHE IRIS DIAPHRAGM IN ASTRONOMY.-In a communi
tion" are duly reckoned and recorded in accordance with cation to the French Academy of Sciences, M. Salet states
approved biometrical methods. It is interesting but not that he has recently and usefully adapted the iris surprising to observe that the frequency curve is usually diaphragm to a telescope in which the magnification em- in fair correspondence with the law of error. ployed is 500. The diaphragm is placed very near to the
The paper ends with a section devoted to general plane of the micrometer wires in front of the field lens,
results." Here we think that too much is made of the and its raison d'étre is to prevent the light from the sky; difficulty of distinguishing between congenital variation and from the illumination of the wires, from reaching the and acquired modification. For practical purposes the even when feeble objects are being observed, the diaphragm distinction is usually obvious enough. A little later the being closed by an external cylinder when the object has
authors observe, The most satisfactory answer to the been brought to the centre of the field. By reducing the question of the hereditary transmission of acquired charextent of the micrometer wires, the diaphragm also re
acters will come as the result of a quantitative (statistical) duces, or eliminates, the effect of astigmatism when
study of variations known to be blastogenic compared with observations of double stars are being made (Comptes a similar study of variations known to be acquired, both rendus, No. 9).
studies to be made on complete series of individuals bred ('ONSTANCY OF
under quantitatively determined life conditions." This SPARK WAVE-LENGTHS.-A question seems to us somewhat like using a steam-hammer to crack which is of first importance to those observers engaged in stellar line-of-sight work, viz. that of the constancy of
an egg. It is not astonishing to find that there is little
or no evidence of differing selection-value in the variable wave-lengths in spark spectra taken under various conditions of discharge, has recently been re-investigated by
number of spots on the elytra of a ladybird; but it hardly Mr. G. W. Middlekauff at the Johns Hopkins University. fact, together with
seems clear that the authors are justified in claiming this A detailed description of the apparatus and methods em
apparent change of “mode"
between the years 1895 and 1901, as evidence in favour ployed, together with the results obtained, appear in No. 2, of vol. xxi., of the Astrophysical Journal.
determinate variation." Before any such inference Mr. Middlekauff used a
can properly be drawn, the question of possible correlation concave Rowland grating of ought at least to be considered. The authors, however, 20,000 lines to the inch and a focal length of 21.5 feet. The self-induction in the spark circuit could be varied from
arrive on the whole subject at the satisfactory conclusion
that natural selection “is after all a logical necessity and 0-00007 to 0.0012 of a henry, and the capacity from 0.0085 undoubtedly an actual actively-regulative factor " in the to 0-0739 of a microfarad, and the results obtained afford formation of species.
F. A. D. strong evidence that in the case of a spark discharge in air, at atmospheric pressure, no
1 "Studies of Variation in Insects." By Vernon L. Kellogg and Ruby
'shift" in wave-length G. Bell, of Leland Stanford Junior University. From the Proceedings of is produced by variations of self-induction or capacity the Washington Academy of Sciences, vol. vi. (Washington, D.C., 1904 )
INTERRUPTERS FOR INDUCTION COILS. any considerable wear are the slate distance-pieces; the IT has been thought that an account of the more im
rim of the fly-wheel may occasionally require truing-upportant forms of interrupter would not be unwelcome
It is important to keep the edges of the brass contactto readers of NATURE.
segments and the surfaces of the slate distance-pieces clean A rotating, air-break interrupter is shown in Fig. 1. | by the occasional application of fine sand-paper. An accurately balanced brass fly-wheel, FW, driven
In Fig. 3 are shown the essential parts of the mechanism by a small motor, is fitted with two insulating segments,
double-dipper." interrupter. The double motoris, let into its periphery. Bearing on the fly-wheel are two
driven crank, c, carries two connecting-rods, CR, each of copper gauze brushes, B, and B,; the circuit is interrupted fixed to the top of a stiff rod, r, which passes between the
which is attached to a cross-head, cH. Each cross-head is as each brush slips over from the brass to the insulating guide-springs, Gs, and through the guide-block, GB. The
latter is supported by a strong bracket, B, screwed to the
stand supporting the motor. Each reciprocating rod ends FW
in an amalgamated copper wire, cw, which dips into the mercury. It will be readily seen that by the adoption of the two-crank arrangement the frequency is doubled for a given speed as compared with the single-crank interrupter; for
while with the latter there is only a single break per revoluus
tion, the former gives two breaks per revolution, one of the
The curve marked “double dipper" in
This type of interrupter is comparatively
cheap and simple, and works very steadily. portion of the rim. It is evident that the arcing which
There is no complicated mechanism to get occurs at the break necessitates the use of a fire-proof in
out of order, and only a small quantity of sulator. A small piece of slate (s in Fig. 1) is fitted imme
mercury is required (about 2lb.). diately behind each brass segment, and this takes the spark; One of the most successful types of rotary it is easily renewed, the remainder of each insulating interrupter is the mercury jet interrupter. segment consisting of vulcanised fibre.
Several varieties of this have been used. One So far as the writer is aware, this type of interrupter of the best known is shown in Fig. 5. The GB was first described by Wadsworth in 1894, and was used vertical motor-driven shaft, S, carries by Prof. Michelson in some Geissler-tube experiments cylinder, c, the lower portion of which is cut (American Journal of Science, pp. 496-501, December, 1894) up into a number of teeth, T. The shafts As might be expected, the suddenness of the break depends is continued downwards, and passes through
the mercury pump casing. The
municates by means of a suction orifice, while above the on the speed of the motor (or frequency of interruption). In mercury is the usual alcohol filling the bulk of the vessel. Fig. 2 are plotted the results of some experiments bearing If in good working order, the mercury jet interrupees on this point. It will be seen that for a given value (root-gives excellent results, as may be seen by referring to the mean-square) of the primary current, an enormously greater curve marked "mercury jet " in Fig. 4. which corresponds spark-length-especially with
with the larger currents-is to a frequency of interruption = 40. A comparison of thes obtained at the higher speed.
curve with that given in Fig. 2 for the rotating air-berak This form of interrupter is not very expensive, and works interrupter at once shows the superiority of the jet inter very satisfactorily so long as the primary current does not rupter.' The mercury jet interrupter is much more expenses exceed about 5 amperes. It shares with the platinum in- and complicated than the “ double-dipper "type, and requirs terrupter the advantage of cleanliness. Renewals and repairs a larger amount of mercury; but it yields somewhat beters cost very little, as the only parts which are subjected to results.