SCIENCE. 



327 



terygian Fishes in the British Museum " (v. 2, p. 316) — j 

 a work which has served as the basis of the "Introduc- 

 tion to the Study of Fishes " — that the " Collichthys 

 pama " inhabits the " Bay of Bengal, entering rivers." 

 The statement given as a deduction is therefore really a 

 co-ordination — and an entirely sophistical one — of the 

 ascertained structural peculiarity and the habitat of that 

 species. 



One other characteristic deduction, also relating to a 

 Sciasnoid type, may be noticed because of its interest to 

 American students. 



The " Drum " of the Atlantic {Pogom'as chromis) 

 is especially mentioned in connection with " the extra- 

 ordinary sounds which are produced by it and other allied 

 Sciaenoids." " It is [says Dr. Giinther] still a matter of 

 uncertainty by what means the " Drum " produces the 

 sounds. Some naturalists believe that it is caused by 

 the clapping together of the pharyngeal teeth, which are 

 very large molar teeth. However, if it be true [sage 

 proviso !] that the sounds are accompanied by a tremu- 

 lous motion of the vessel, it seems more probable that 

 they are produced by the fishes beating their tails against 

 the bottom of the vessel in order to get rid of the parasites 

 with which that part of their body is infested." In this 

 paragraph are several illegitimate assumptions and 

 inferences which a slight knowledge of the literature re- 

 specting the subject would have prevented. (1) The 

 sounds are entirely independent of ' vessels." (2) There 

 was no reason to suppose that the fish in question was 

 more infested with parasites in the tail than any other. 

 (3) The statement that " allied Sciaenoids " (and this is 

 especially true of the closely related fresh-water sheeps- 

 head, or Haploidonotus, referred by Giinther to a genus 

 with which it has not the slightest affinity!) produce 

 similar sounds was for the moment forgotten. (4) The 

 co-ordination of facts and phenomena rendered it un- 

 necessary to look to such source for solution. (5) The 

 source indicated was one of the most improbable that 

 could be conceived. There is, indeed, ample cause for 

 surprise that any educated ichthyologist could suppose 

 that a fish would agitate its tail in the manner suggested 

 to relieve a spasmodic pain, such as is postulated by the 

 explanation given. Our author's credence in the allega- 

 tion that the sounds produced are " accompanied by a 

 tremulous motion of the vessel," was, as we have seen, 

 sufficient to impel him to substitute a most improbable 

 for at least a probable hypothesis. 



A mistake of another kind is made respecting the Rays. 

 It is said that "the majority are oviparous" (p. 336). 

 As was long ago recognized by Mtiller and Henle, the 

 Raiidae are the only oviparous rays ; Gunther includes 

 them all in one family and four genera, and admits about 

 35 species. All the others recorded by him, so far as 

 known, are viviparous ; they number, in his opinion, five 

 families, twenty genera and more than 100 species, conse- 

 quently the majority are viviparous ! 



Whether a work so abounding in errors that we are 

 only able to specify a few as examples and hint at sdrne 

 kinds of others is worth acquiring must be left to the 

 reader to judge. As a curiosity in taxonomical literature 

 it certainly is, but for such purposes as are most desirable 

 — correct information and identification of genera — it as 

 certainly is not, Theo. Gill. 



Descartes and the Barometric Theory. — At one of 

 the late sittings of the Academy of Moral and Political 

 Science, M. Nourisson made an extremely interesting com- 

 munication lelative to a letter of Descartes, in which the 

 great philosopher clearly indicates the principal of atmos- 

 pheric pressure, twelve years before Toricelli's experiments 

 on the barometer. Toricelli constructed the fast barometric 

 tube in 1643 ; in 1647 Pascal accomplishes his celebrated 

 experiments of Puy-de-D6me and of the "Tour Saint 

 Jaques." It would appear that Descartes had suggested to 

 the author of Pensics the idea of this mode of experiment. 



CORRESPONDENCE. 

 To the Editor of Science : 



ON ETHER. 



There are two theories in regard to ether, one of 

 which assumes that it is a discontinuous medium, that is, 

 a medium composed of particles at enormous distances 

 apart, as compared with their diameters. 



In this theory ether is spoken of or defined as an " im- 

 ponderable elastic medium." If we examine the above 

 definition we find several inconsistencies. To begin 

 with, an imponderable body is a body without weight. 

 Now the weight of a body, is the result of the mutual 

 attraction, exerted between it and some other body ; in 

 other words, weight is the effect of gravitation. Now 

 as every particle attracts every other particle with a force, 

 that is directly as the mass, and inversely as the square 

 of the distance between them, an imponderable body 

 must be one in which the mass is zero, or that is at 

 such a distance from every other body that the re- 

 ciprocal of the square of this distance is zero. The last 

 supposition is of course absurd. 



Now the mass of a body is equal to the product of its 

 volume and density, or M—d V and if M is equal to 

 zero, either d or V must be zero and as it would be im- 

 possible to conceive of a body that occupies no space, we 

 must think of d as equal to zero, or in other words an 

 imponderable body is simply a portion of space. This 

 same theory assumes that radiant energy is transmitted 

 by means of the moving particles of ether, i. e., one 

 particle moving with a certain velocity, strikes another 

 and imparts some of its energy to it and this flying off 

 strikes another and so on. But the momentum of a 



MV t 



body is expressed by M V and its energy by — - — 



( V— Velocity), making M equal to zero, as we must if 

 the particles are imponderable, we have O V=M a =o 



O V' 1 



and =E=o, hence the transmission of radiant 



2 



energy by an imponderable substance, composed of 

 particles is an impossibility. If we assume that the 

 particles are effected by gravitation, then at once it is 

 evident that the ether could not be of equal density 

 throughout the universe, for around each celestial body 

 there would be an atmosphere of ether which would 

 gradually decrease in density from the surface of the 

 body outwards. 



By elasticity in the above definition, is meant that 

 property of matter, possessed by gases in the highest de- 

 gree, of having its volume or density changed by some 

 force and regaining its former state when the original 

 condition are again imposed. When a gas is compressed, 

 the mean free path of the molecules is shortened and 

 the compressibility is dependent upon the length of the 

 mean free path. When the pressure is removed, the gas 

 expands, the expansion being due to a conversion of the 

 energy of vibratory motion of the molecules or heat into 

 energy of translation. If the ether is elastic, then of 

 course with a change from less to greater density the 

 particles must be moved nearer together, and the com- 

 pressibility will be dependent upon the average distance 

 between the particles. When a change from greater to 

 less density takes place, the particles must be moved 

 farther apart and the explanatory reason given for this 

 expansion is that the energy of the moving particles 

 causes the expansion. 



From what has been said in regard to imponderability, 

 it is evident that a discontinuous imponderable elastic 

 substance is an impossibility according to the present 

 ideas of dynamics. The transmission of radiant energy 

 by a discontinuous ether, if the particles are ponderable, 

 is possible in two ways, 1st, By an alternate rarefaction, 

 and condensation of the ether, similar to the manner in 

 which sound is transmitted through air. 2d, By the 



