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, as 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 ; 1 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 
