PKOCEEDINGS 
OF THE 
ROYAL SOCIETY OF EDINBURGH. 
VOL. XII. 1883-84. No. 117. 
Monday, March 1884, 
EOBEET GEAY, Esq., Vice-President, in the Chair. 
The following Communications were read : — 
1. The Old Eed Sandstone Volcanic Eocks of Shetland. By 
Messrs B. N. Peach and John Horne, of the Geological 
Survey of Scotland. 
2. On the Principles of Economics. Part I., Mathematical. 
Part II., Physical. By Mr P. Geddes. 
3. An Integrating Hygrometer. By Prof. C. Michie Smith. 
This instrument consists essentially of a cryophoriis, one bulb of 
which is kept wet by the same arrangement as is used for a wet- 
bulb thermometer, while the other bulb is left dry. For con- 
venience, the wet bulb is made spherical, and the dry bulb is made 
cylindrical, and is graduated. The difference of temperature be- 
tween the two bulbs produced by evaporation causes a transference 
of water from the dry bulb to the wet bulb, and the amount of 
water thus transferred in any period of time, taken along with the 
mean temperature, will give a measure of the average hygrometric 
2 Q 
VOL. XII. 
