136 
Proceedings of the Royal Society 
summer solstice; lightning (only) has its maximum period during 
that time of the year when the humidity of the air is at its 
maximum; hail is most frequent during that period of the year 
when the temperature is rising, or when the vertical layers of the 
atmosphere is in most unstable equilibrium ; and snow during the 
coldest months of the year, with this striking peculiarity, that 
the maximum period is not in the depth of winter, but in March, 
in the end of winter; immediately after which the curve abruptly 
falls. 
The intimate connection of the thunderstorm with summer 
rainfall, and the important bearing of the whole four curves on 
climatology, was referred to. 
2. Note on the Origin of Thunderstorms. By Prof. Tait. 
This Note does not refer so much to those great thunderstorms 
which extend over hundreds of miles in each direction, as to those 
small local storms which are often seen of from two to five or 
six miles only in diameter. 
It refers particularly to those which are seen, in summer and 
autumn, to pass down the Tay valley. They almost invariably 
come from the westward, and I am told each is almost always 
accompanied by a storm of similar dimensions passing eastwards 
down the valley of the Forth. So far as I can ascertain, they 
seem both to commence almost abruptly somewhere in the district 
about Ben Ledi and Ben Lomond. 
Seen from St Andrews, which they frequently pass at a few miles 
distance to the northward, they usually appear to be in a state of 
rotation about a vertical axis. It is not very easy to judge of the 
relative distances of the various clouds, so as to ascertain the sense 
of the rotation ; but, in one case which I observed carefully last 
autumn, the rotation appeared to be in the positive direction, — i.e ., 
opposite to that of the hands of a watch whose face is turned 
upwards. 
If this be generally the case, and if it should be found that the 
direction of rotation of the companion storm in the Forth valley 
is negative , it would seem that their common origin may be 
explained on the following very simple hypothesis, which has the 
