532 
Proceedings of the Royal Society 
in Aberdeenshire. It is a fine-grained, rather soft rock, containing 
scattered cubes of pyrites, and capable of being readily dressed into 
thin smooth slabs. A tombstone of this material, erected in the 
old burying-ground at Peterhead, sometime between 1785 and 1790, 
retains its lettering as sharp and smooth as if only recently incised. 
Yet the stone is soft enough to be easily cut with the knife. 
The cubes of pyrites have resisted weathering so well, that a mere 
thin film of brown hydrous peroxide conceals the brassy undecom- 
posed sulphide from view. The slate is slightly stained yellow 
round each cube or kernel of pyrites, but its general smooth surface 
is not affected. The lapse of nearly a century has produced scarcely 
any change upon this stone, while neighbouring tablets of white 
marble, 100 to 150 years old, present rough granular surfaces and 
half-effaced though still legible inscriptions.] 
2. On a Realised Sulphurous Acid Steam-Pressure Ther- 
mometer, and on a Sulphurous Acid Steam-Pressure 
Differential Thermometer. By Sir William Thomson. 
A sulphurous acid steam-pressure thermometer, on the plan 
described in my communication on the subject to the Royal 
Society of March 1, has been actually constructed, with range up 
to 25° C., but not yet in a permanent form. The slight trials 
I have been able to make with it give promise that, in respect to 
sensibility and convenience for practical use, it will most satis- 
factorily fulfil all expectations, and have given some experience in 
respect to the overcoming of difficulties of construction, from 
which the following instructions are suggested as likely to be 
useful to any one who may desire to make such an instrument : — 
(1.) The sulphurous acid steam thermometer might more pro- 
perly be called a cryometer than a thermometer, because it is not 
very convenient, except for measuring temperatures lower than the 
atmospheric temperature at the place and time of observation ; for, 
it must be remarked, that the thermometric substance, that is to 
say, the infinitesimal layer of liquid and steam of sulphurous acid 
at the interface between the two in the bulb in the annexed 
drawing (fig, 1), must be at a lower temperature than any other 
part of the space of bulb and tube between it and the mercury 
