SCIENCE. 



463 



with the aid of his scholars is oftentimes the most useful 

 for purposes of instruction. Many and many a school 

 has invested in trifling electrical playthings a sum of 

 money which would have gone far towards the establish- 

 ment of a simple working laboratory. 



" In physics the laboratory practice must needs be some- 

 what limited. The pupils may handle whatever appa- 

 ratus happens to be available, learn its manipulation, and 

 assist the teacher in the construction of simple appli- 

 ances. The magnetization of needles, the electrolysis of 

 liquids, the verification of the fixed points upon a ther- 

 mometer, and rough determinations of specific gravity, 

 boiling point, and melting point are among the many 

 experiments which ought always to be possible. 



" In the chemical laboratory a much greater variety of 

 work is easily attainable. There are the ordinary experi- 

 ments in manipulation, such as the bending of glass 

 tubes, filtration, precipitation, distillation, &c, the 

 preparation of the commoner gases, acids, and salts ; the 

 verification of the more obvious properties of the chemi- 

 cal elements ; and lastly, the simpler reactions of qualita- 

 tive analysis. To the last named subject some time may 

 always be profitably assigned. No other class of exer- 

 cises will do so much towards impressing the average 

 beginner or towards making him realize the nature of 

 chemical reasoning. At every step it calls his powers of 

 judgment into play. It involves the use of no costly 

 apparatus, and enough can be done for all school pur- 

 poses with a very moderate supply of the cheaper chemi- 

 cals. At an expense of a hundred dollars a year a great 

 deal can be accomplished ; and an outlay of only one- 

 fifth of that sum may yield results which are by no means 

 to be reckoned as trivial. Again let it be said that suc- 

 cess depends upon the teacher and not upon the cost of 

 materials." 



We shall in our next issue continue our notice of 

 this interesting report. 



ON COMETARY APPEARANCES. 1 

 By M. Jamin. 



[Translated from the French by the Marchioness Clara Lanza.] 



The question of comets seems at present to occupy the 

 attention of all savants, and as M. Faye has prevailed 

 upon physicists to take up the subject also, I have de- 

 cided to enter into the discussion, not with the intention 

 of creating any novel hypothesis, but rather to oppose 

 that which M. Faye imagines to be the correct one. In 

 the first place, it appears to me unnecessary. It contra- 

 dicts in my opinion, the theory of the vibration of the ether. 

 Besides, it deprives the law of gravitation of its general- 

 ity and simplicity. In his first work, M. Roche deter- 

 mined, by means of calculation, the form of the horizon- 

 tal strata of cometary atmospheres subject to the sun's 

 attraction, but he omitted to note the variations of tem- 

 perature occasioned by the solar rays on the two sides of 

 the comet. In this way he was led to think that the lat- 

 ter must have two tails, one turned towards the sun, the 

 other away from it, which supposition is contrary to real- 

 ity, as it should be, in fact, inasmuch as it overlooks the 

 cause which manifestly determines the unsymmetrical 

 forms of the two sides. In a second paper, however, he 

 makes a correction, by supposing the existence of a re- 



'See Comptes Rend 11s, August 16, 1881. 



pulsive force diminishing the solar attraction about 

 I — to 1. <p, </>, being a force acting unequally upon different 

 substances, and which is in reverse ratio to their density. 

 This hypothesis admits of the calculation being achieved 

 with facility, but it has no physical actuality. It is con- 

 fined to replacing the warmth of the cometary atmosphere, 

 which should be included in the calculations, but which 

 has been forgotten, with a wholly imaginary action whose 

 existence no experiments have ever confirmed. I shall 

 endeavor to re-establish the effect due to the unequal 

 warmth of the two sides by referring to analogous 

 conditions which should exist between the Earth and 

 comets. 



Upon the Earth, every day throughout the year, the solar 

 rays one after the other in regular succession strike normal- 

 ly all the points of a circle perpendicular to the axis of rota- 

 tion and near the equator. These points on all portions 

 of the globe are those that receive the maximum of heat 

 at noon. They constitute what is termed the ring of 

 aspiration. The air there really becomes rarified and 

 ascends, advancing towards the north or the south, as the 

 case may be, determining two gaseous currents called 

 trade winds. These currents are permanent and regu- 

 lar ; they come from temperate climates, grow warmed 

 progressively in their course, carry with them an intense 

 evaporation, are slightly deviated towards the west in 

 consequence of the Earth's rotation, and finally meet 

 obliquely upon the ring, to rise to the highest atmospheric 

 limits. There they spread, then taking a contrary course, 

 return, one towards the north, the other towards the 

 south. These are the counter trade winds. There are, 

 therefore, on the two sides of the ring of aspiration, two 

 closed atmospheric currents completely enveloping the 

 globe, coming cold from the poles, grazing the Earth, 

 and then returning warm, by a higher route. There is 

 no occasion to dwell upon the chief rdle played by this 

 circulation. It is sufficient to merely demonstrate its 

 necessity, its constancy and its extent, besides recalling 

 the theory due to the famous Halley, which has never 

 been contested. 



This circulation would still exist although under 

 changed conditions, if the Earth instead of turning on 

 its axis always presented the same side to the Sun. The 

 ring of aspiration would be reduced to a single point, the 

 trade winds would converge in all directions, while the 

 counter winds would diverge in the same way. All 

 points of the Earth would send to this summit cold air 

 which would grow warm there, rise in the form of a cone 

 toward the Sun, spread, become bent upon the edges like 

 the chalice of a cvrathiform flower, leave the Sun by 

 a high route and after a more or less prolonged journey 

 would return to the point of departure grazing the 

 Earth's surface. It is very evident that this double 

 movement would possess an increase of force propor- 

 tionate to the Earth's approach to the Sun ; that its at- 

 mosphere would be more extended, and that there would 

 be a greater mass of water to be evaporated. This does 

 not imply any particular repulsive force. 



But let us get to the comets. In the long journey 

 which they perform slowly until they are beyond the 

 Solar world, they have plenty of time to lose all the heat 

 received from the Sun, and to efface all traces of pertur- 

 bation. The tail disappears, the matter is knit together 

 by its own attraction and assumes a nebulous, spherical 

 form. In the centre are the dense, solid substances, the 

 nucleus, then the liquids and finally the gases. An 

 enormous atmosphere and a very small nucleus. In 

 Donati's comet the nucleus measured 1600 km., while the 

 atmosphere was 20,000 km. The comet of 1881 was 

 still more extraordinary. Its aureole measured 2,000,000 

 km., the nucleus was reduced to 680. This is just con- 

 trary to the Earth whose diameter amounts to 12,000 

 km., while its atmosphere is merely a 1 hin pellicle of 18 

 or 20 leagues. Comets are so constituted that the most 

 tremendous atmospheric movements are developed be- 



