300 
Fahrenheit's thermometer. Alcohol, with its vapour, for 
temperatures from about 90° to 180°. Water, with its vapour, 
for temperatures from about 180° to 212°; other fluids may, 
however, be used. When the Thermostat is fixed in such 
structures as hothouses, he prefers that it should be shaded 
from the direct rays of the sun. He also exhibited the 
following table of the tension of aether vapour at various 
temperatures, and of the pressure of the atmosphere in 
pounds, on each acting square inch of the apparatus : — 
Temperature. 
34° 
44° .... 
54° 
64° 
74° 
84° 
94° 
104° 
Tension of va- 
Pressure of At- 
pour in inches 
mosphere on appa- 
of Mercury. 
ratus in lbs. 
. 6.2 
, 8.14 
■ 10.93 
. 10.3 
9.85 
, 13 
8.5 
, 16.1 
6.95 
, 20 
, 24.7 
2.65 
. 30 
0. 
Mr. Pearsall, of Leeds, said a few words on the subject, 
and referred to the vast importance of such an invention as 
that which had just been described, and especially to its in- 
troduction into hospitals, and for a variety of useful purposes. 
It was capable of very important results. 
ON THE ORIGIN OF SLATY CLEAVAGE. BY H. C. SORBY, ESQ. 
F.G.S., OF SHEFFIELD. 
Before describing the theory I have propounded to account 
for what is called " slaty cleavage," it will probably be the 
best for me to explain briefly the nature of this structure and 
of the problem to be solved. Most stratified rocks can be 
split with more or less facility in the plane of their bedding, 
but in certain districts, greatly disturbed by elevatory forces, 
besides this line of weakness, there occurs another, only 
