992 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 
observations with two or more thermometers) this quantity may be very readily and accu- 
rately determined ; and it affords the only unexceptionable manner of ascertaining the conduc- 
tivity of the earth’s crust on a large scale. 
* * *% * 
“ The epochal retardations for the annual curves at the depth of a few feet, follow, gene- 
rally speaking, a simple law, for they are propagated uniformly downwards with a velocity 
which is easily connected with the constants proper to the soil, determined from the range at 
“two given depths, as just explained. It must not be concluded, however, that the epochs of 
earth-temperature at the surface coincide with those of air-temperature in the adjoining 
stratum. The difference of epoch may be obtained in terms of the conductivity and superfi- 
cial characters of the solid stratum. But the complete expression for the epoch at any depth 
in terms of the dates of maximum and minimum at some other depth, and of the constants of 
conductivity and surface, derived from two observed ranges, is so complex, that, so far as I 
know, no attempt has been made to verify M. Porsson’s formule except in a single example 
by himself, taken from M. ARAGO’s observations.” 
