528 
Monday , 3 d February 1862. 
Dr CHRISTISON, V.P., in the Chair. 
The following Communications w r ere read : — 
1. Investigation of an Expression for the Mean Temperature 
of a Stratum of Soil, in terms of the time of year. By J. D. 
Everett, M.A., Prof, of Mathematics, &c. in King’s College, 
Windsor, Nova Scotia. Communicated by Prof. Tait. 
In vol. xxii. of the Transactions of the Society, Prof. W. Thom- 
son has given (after Fourier) the general solution of the problem of 
underground conduction, and he and Professor Everett have given 
methods more or less accurate of determining from observation the 
specific constants for any locality. These are mainly applied to the 
results of Principal Forbes’s observations on the Calton Hill and 
other places near Edinburgh. (Trans., vol. xvi.) 
The present paper has for its object the calculation of the mean 
temperature of a given stratum of soil at any given time. If v be 
the temperature at time t, at depth x, v is known by the paper first 
referred to, in terms of x and t — and the object of the present paper 
is the easy approximate calculation of the value of the quantity 
^7 — between any limits of depth x. The method is applied to the 
Jdx 
Calton Hill observations. 
2. Notice of the Catadioptric Altitude and Azimuth Circle. 
By Edward Sang, Esq. 
This little instrument differs from the ordinary theodolite in the 
means by which the direction of the object under examination is 
indicated. 
The simplest contrivance for marking the direction of a distant 
object is a straight rod. This, with the very obvious improvement 
of studs at the ends, was the only instrument available to the 
ancients for astronomical observations ; it is yet employed in gun- 
nery, and even by surveyors in the plain-table and cross-staff. 
The objection to the plain-sight, when it is to be applied to 
