ON UNDERGROUND TEMPERATURE. 511 
The annual range of temperature diminishes in the ratio of 2 or 3 to 1 for 
every 10 feet of descent, and becomes reduced to a tenth of a degree Fahr. 
at depths of from 50 to 80 feet in this climate,—these results being derived 
from observations extending to 25 feet of depth, made at Greenwich and at 
three localities in Edinburgh. 
A great boring or excavation, such as a mine, necessarily produces much 
disturbance of the normal temperature in its neighbourhood, and therefore, 
while observations in mines ought not to be neglected, we think the efforts 
of the Committee should be chiefly directed towards finding the temperatures 
at various depths in smaller borings, such as are usually made preliminary to 
mining-operations. As these are often carried to depths of from 300 to 600 
feet, they will furnish very measurable differences of temperature at different 
depths in the same boring. It is suggested that observations should be made 
at every 50th foot of depth. E 
A method which has been used by Angstrém will probably be found most 
convenient. It consists in enclosing a thermometer in a large glass bottle of 
water, letting it down to the point where the temperature is to be taken, 
leaving it in that position long enough to ensure that its temperature shall 
not differ sensibly from that of the soil nearest to it, then drawing it quickly 
up and reading off the thermometer before time is allowed for any sensible 
variation in its temperature. It would probably be found advisable to use 
two pistons or plugs (two bags of sand might answer the purpose), one above 
and the other below the bottle, to check currents of air or of water. 
The thermoelectric method might also be followed with great advantage. 
Two wires, one of iron and the other of copper, insulated by gutta percha or 
some other covering as in submarine cables, and connected at their ends, 
might be let down, so as to bring their lower junction to the point where the 
temperature is to be taken, their upper junction being immersed in a basin 
of water, and the circuit completed through a galvanometer. The tempera- 
ture of the water in the basin might then be altered till the galvanometer 
gave zero indication. An extremely accurate determination of the tempera- 
ture at various depths could in this way be obtained with great ease and 
expedition, when the apparatus had once been prepared; but the method by 
water-bottle, though requiring more time for the observations, will probably 
be in general preferred on account of its simplicity. 
Currents of water in a boring will render it unsuitable for our purpose ; 
but water free from currents will but little affect the accuracy of observa- 
tion. 
Every Member of the Committee is requested to find out borings, in his 
_ own neighbourhood or elsewhere, that would be suitable for the investigation, 
and also to state whether he could undertake to make the observations him- 
self, the thermometers or other apparatus required being supplied by the 
Committee. 
Sir Wm. Thomson has already ordered from Casella two thermometers 
_ suitable for the water-bottle method, and expects to have them almost imme- — 
_ diately for trial; they are spirit thermometers, with Fahrenheit scales, and 
_ are to be accurate toa tenth of a degree. 
Your reply at your earliest convenience, with any suggestions you may 
eve tovoffer, will oblige Your obedient Servant, 
J.D, Evrrert, 
Queen’s College, Belfast, Nov. 16, 1867. Secretary to the Committee. 
