SCIENCE 



NEW YORK. FEBRUARY 84, le 



generally or ever reach. Hence it is that while the earth can 

 retain the atmosphere with which it was endowed, the moon is 

 unable to do likewise. 



THE ABSENCE OF AIR FROM THE MOON 



BY SIR ROBERT BALL, LOWNDEAN PROFESSOR AT CAMBRIDGE, 

 ENGLAND. 



Astronomers have long felt that the absence of air from the 

 moon is a fact that demands some special explanation. Most of 

 the globes in space which are known to us are encompassed by- 

 more or less copious atmospheres, vvhy then is the moon an ex- 

 ception ? why should there be a gaseous investment to the earth 

 and to Venus, to Mars and to Jupiter, and why should the moon 

 alone be de^foid of such cohering? The sun and other stars are 

 also so very copiou'ily endo.ved with gaseous surroundings that 

 the total want of anything of the bind from the moon becomes 

 all the more enigmatical. 



At last a light has been thrown on the matter, and an explana- 

 tion is now provided which is so consonant with the present state 

 of physical knowledge, that I cannot hesitate to accept it. The 

 absence of air from the moon is a necessary consequence of the 

 kinetic theory of gases. 



According to the principles of this theory, now generally ac- 

 cepted among physicists, any gas such as oxygen or hydrogen, 

 is composed of molecules which move with an extreme degree of 

 rapidity. The molecules of hydrogen, for instance, which are 

 the most nimble of all the gases in their movements at ordinary 

 temperatures, dash along so fast as to travel on the average 

 somewhat mire than 6,000 feet a second. Oxygen and nitrogen 

 have mnvemonts which are generally much less than those of 

 hydrogen. But it is to be noted that, in the course of their move- 

 ments, individual molecules freq lentlv attain velocities very 

 much in excess of the average pace. This is the important point 

 for our present purpose, for on it depends the explanation of the 

 phenomenon of which we are in search. 



It can be shown that the mass and the dimensions of the moon 

 are such that if a body were pr.ijected upwards from its surface 

 at a pace, let us say, of half a mile a second, that body would 

 ascend to a very considerable elevation, ultimately, however, the 

 attraction of the moon would overcome its outward movement, 

 and the body would tumble back again. If, however, the initial 

 pace were so much greater that it attained a certain critical 

 amount of about a mile a second, then the missile, according to 

 the laws of motion, would ascend from the surface of the moon 

 and go on and on never to be again re-called by any power that 

 the moon's attraction could put forth. 



Let us suppose that the moon were now to be invested with a 

 new atmosphere of oxygen or nitrogen. The molecules of these 

 gases will of course, be darting about with the velocities appro- 

 priate to their nature, but, generally speaking, the speeds with 

 which they are animated remain wiihin the limits of velocity 

 which it is in the power of the moon to control. But these are 

 only the average speeds, and it will frequently happen that indi- 

 vidual molecules will be animated by a speed equalling or exceed- 

 ing the critical pace of a mile a second; if this takes place at the 

 upper layers of the moon's atmo'-phere, the little molecules will 

 take leave of the moon altogether. Oiher particles follow in the 

 same fashion, and thus it happens that an atmosphere composed 

 of such gases as these we know could not permanently abide on 

 the moon. 



On the earth we have and we retain a copious atmosphere. 

 The reason simply is that the earth is massive enough to require 

 that a projectile shall attain a speed of about six miles a second 

 before it goes off and takes leave of our globe. This velocity it 

 would seem that the molecules of oxygen and nitrogen do not 



SOME ERRORS IN THERMOMETER READINGS. 



BY FRANK WALDO, PRINCETON, N. J. 



I HAVE understood that the long-awaited comparison of ordinary 

 thermometers with the gas thermometer, at very low tempera- 

 tures, has been carried out at the International Bureau of Weights 

 and Measures at Sevres. However, I have been unable to get 

 hold of any account of this work, as the ofBcial reports concern- 

 ing it had not been received a short time ago even at the Weather 

 Bureau Library. In Wild's Bepertorium fur Meteorologie, Vol. 

 XV., which has just been received, there is an account of some 

 careful comparisons ac low temperatures, which gives results 

 probably not very different from those obtained at Sevres ; and a 

 little summary of this will undoubtedly be of interest to some 

 readers. In the St. Petersburg paper,' S. Hlasek gives a little 

 summary of the condition of the thermometric standards of the 

 Russian Meteorological Service from the time Director Wild took 

 charge (about 1868) up to the present time. In the present com- 

 munication, I will not trace through the various thermometer 

 corrections as given by Hlasek, but will merely give the latest 

 results, showing the corrections to be applied to the standard 

 mercurial thermometer at moderate and low temperatures and to 

 the standard spirit thermometer at very low temperatures, to re- 

 duce them to the hydrogen gas thermometer, which is the inter 

 national standard. 



These corrections were obtained by means of a normal (Toluene) 

 thermometer, Tonnelot No. 4932, which had been compared with 

 the hydrogen gas thermometer at Sevres. 



' Die Temperatur-soalen des Pliyslkal. Cent. Observ. und ilir Verliiiltness 

 za der International Temperatur-scale, 1892. 



