40 



supposition that there is matter in space in the form of gas, 

 and that the comet causes it to be electrically illuminated 

 by a brush, ae I stated in my former communication. 



Again, if the tail of the comet be electricity of one kind 

 (say negative), leaving the comet never to return, then the 

 comet must leave the neighbourhood of the sun with a 

 charge of positive electricity, which, as it gets further from 

 the sun and evaporation becomes feeble, will in time over- 

 power the negative electricity in the atmosphere, which will 

 then be attracted by the sun instead of repelled, and if the 

 comet has any tail it will now turn away from the sun ; 

 in which condition it will probably remain until its approach 

 to our sun or some other star again cause it to become 

 negative and turn round. In this case a periodic 

 comet would turn its tail round at definite points in its 

 orbit, and owing to the lagging of the symmetri of the 

 comet's appearance in its orbit the point of turning will 

 be nearer to the sun on its return than on its departure. 

 Now, it seems from a remark of Professor Airy that comets, 

 when first seen, often have their tails before them, and 

 that such is the case with Encke's comet now visible. 



"On the Rupture of Iron Wire by a Blow," by John 

 HoPKiNSON, B.A., D.Sc. 



The usual method of considering the effect of impulsive 

 forces, though in most cases very convenient, sometimes 

 hides what a more ultimate analysis reveals. The following 

 is an attempt to investigate the effect the blow of a moving 

 mass has on a solid body in one or two simple cases ; I ven- 

 ture to lay it before the Society on account of its connexion 

 with the question of the strength of iron at different tem- 

 peratures. 



I assume the ordinary laws concerning the strains and 

 stresses in an elastic solid to be approximately true, and 

 that if the stress at any point exceed a certain limit rupture 



