﻿236 Notices respecting New Books. 



t ached from the dark body of the moon, and many degrees distant 

 from it. It ought also to increase in brightness from its inner border 

 for a considerable distance outward. 



Others have imagined that the corona might be attributable to the 

 passage of the sun's light through a lunar atmosphere ; but since 

 some of the streamers, or rays of the corona, have been seen to ex- 

 tend to a distance greater than the sun's diameter, this would require 

 the lunar atmosphere to be of vast extent ; whereas no decisive evi- 

 dence has yet been obtained of the existence of any lunar atmosphere 

 capable of producing a sensible refraction, or reflecting a perceptible 

 amount of the sun's light to an observer on the earth. 



Perhaps the more prevalent idea, at the present day, is that the 

 corona, with its rays and tufts of light, is a phenomenon of diffraction 

 produced by the passage of the sun's rays along the denticulated 

 edge of the moon. This theory has an air of plausibility, but it is 

 entirely inadequate to account for the great extent of the coronal 

 rays. The fringes produced by the diffraction of light in its passage 

 near the edge of a body appear to the eye of the observer to extend 

 but a small angular distance from the edge. This would be more 

 strikingly true in the case of a distant body, like the moon. 



The only remaining supposition is, that the corona is either an 

 envelope of some kind permanently connected with the sun, or is 

 made up of material emanations proceeding immediately from the 

 sun. To the large body of indirect evidence that has been obtained 

 that the corona is wholly a solar phenomenon, we may now add 

 that of direct observation, since it appears that " an examination of 

 the photographs of totality," obtained at the eclipse of 1869, shows 

 that as the moon advanced the corona was progressively covered. 



XXX. Notices respecting New Books. 



Principles of Mechanism, designed for the use of Students in the Uni- 

 versities, and for Engineering Students generally. By Robert 

 Willis, M.A., F.R.S., Jacksonian Professor of Natural and Ex- 

 perimental Philosophy the University of Cambridge, %c. Second 

 Edition. London: Longmans, Green, and Co. 1870. 



THE distinction between motion considered merely as change of 

 place, and motion as produced by force, was often noticed by 

 the earlier writers on mechanics, e. g. by Euler ; and the possibility 

 of establishing a distinct science of pure motion was pointed out by 

 more than one writer, and particularly by Ampere, who denominated 

 it Cine'matique or Kinematics. It is, however, worthy of remark that 

 there are two points of view from Avhich such a science may be re- 

 garded, viz. either, first, in its most general aspect, as a science which 

 treats of the transference of a collection of points from a given posi- 

 tion to any other position, and discusses the formulae which define 

 the positions of these points after and during transference with re- 

 spect to their initial place ; or, secondly, as a science which enumerates 

 and classifies the means by which we change the direction and velo- 



