NATURAL PHILOSOPHY. 169 



proved by the speaker that this view is wholly untenable. One of its con- 

 sequences would be that a bubble of air would be capable of absorbing in a 

 few minutes a quantity of heat which would raise its temperature upwards 

 of 400,000 degrees, or more than 160 times that of fused cast iron. The 

 melting of the ice was shown to be a simple consequence of the dynamical 

 theory of heat ; molecular motion is transmitted through the solid ice, with- 

 out prejudice to its solidity, and detaches the particles at the surface of the 

 Internal cavity, as the last of a series of elastic balls is detached by force 

 which has traversed a row of them without producing visible separation. 

 The passage of snow into glacier ice was next considered. It was referred 

 to the enormous pressure of the moist neve upon its own mass'. That mois- 

 ture was necessary was shown by moulding ice at thirty-two degrees into 

 cups ; while, when it was rendered perfectly dry by immersion in a bath of 

 solid carbonic acid and ether, the ice on being crushed became a powder as 

 white as snow. Crushed glass or quartz could not have been whiter or more 

 opaque. 



OX THE EXISTENCE OF AX ETHEPtEAJL MEDIUM. 



At a recent meeting of the French Academy, M. Le Terrier was asked 

 (on the occasion of announcing the return of Encke's comet in almost ex- 

 actly the position assigned to it by calculation) the very delicate and difficult 

 question, whether or not the present rate of return of this important little 

 body confirms the opinion which the study of its orbits originated, namely, 

 that the space through which the heavenly masses move, is full of a resisting 

 ethereal medium, tending always to retard their advances, diminish their 

 orbits, and bring them finally into the sun. The reply was entirely non- 

 committal, and, in brief, was that, even if Encke's first suspicions of it should 

 not be confirmed, his long and admirable calculations of the subject of this 

 resistance shows the handling of a master. Mr. Babinet, however, boldly 

 entered upon the subject, and asserted that the continued observations of this 

 comet teach us, as yet, nothing definite upon the subject. 



PERSONAL EQUATION. 



At a recent meeting of the Royal Astronomical Society, England, the 

 Astronomer Royal read the following communication from Professor Mitchel 

 of Cincinnati, on " Personal Equation." 



" At the meeting of the American Association of 18-30, I announced the 

 fact that I had contrived a simple apparatus for the investigation of the 

 subject of ' absolute personal equation/ Sickness in my family, and other 

 causes, have combined to prevent a full investigation of this subject until 

 the beginning of the present year. In case a star could be made to record 

 the moment of its own transit, the difference between the star's record and 

 that of any observer would be the observer's absolute personal equation, or 

 what I shall term hereafter the ' personality of the eye.' In like manner, in 

 case a sound could be made to record the moment of its occurrence, the dif- 

 ference between the record and that of an observer would give what I term 

 ' absolute personality of the ear.' The same of the sense of touch, which 

 as a matter of physiological curiosity, has somewhat engaged my attention. 

 As the stars cannot at present be made to record their own transits, I have 

 substituted what I call 'artificial stars,' moving uniformly with a velocity 

 somewhat greater than that of an equatorial siar observed \\iih u jxjwor of 



15 



