40 
times were equal, so were the velocities equal, for the 
successive discharges up to 6 atmospheres. 
The velocity for low pressures, as I have shown in 
Table III., is compounded of the rate of discharge into a 
vacuum and the resistance of the atmosphere, and approxi- 
mates to the square roots of the pressures. For effective 
pressures below 11b. above the atmosphere, the rates of 
discharge are as the square roots of the pressures, as has 
been shown by Dr. Joule in the paper previously referred 
to. 
That the phenomenal rates of discharge which I have 
described are manifested whenever slight differences of 
pressure exist between the discharging and receiving atmos- 
pheres, may be inferred from the familiar experiment of 
fixing a perforated disk of cardboard by its centre to the 
end of a small metal tube, or a piece of tobacco pipe : when 
a similar plain disk, placed on, or against the other, instead 
of being driven off by a jet of air blown through the pipe, 
is attracted to it. 
MICEOSCOPICAL AND NATURAL HISTORY SECTION. 
Ordinary Meeting, October 12th, 1885. 
Thomas Alcock, M.D., President of the Section, 
in the Chair. 
Prof. Boyd Dawkins, F.K.S., brought before the notice of 
the Section rock-specimens and microscopic slides illustrat- 
ing the structure of the clay-slate of Snaefell in the Isle of 
Man. The day-slate had in some places been subjected to 
enormous lateral pressure by which the lamination-planes 
