Page:A Treatise on Geology, volume 2.djvu/287

 CHAP. IX. , has made some experiments in the comparatively shallow salt mines of Cheshire which evince an augmentation of 1° in 70 feet from the surface.

But the greatest strength of observation, independent of mineral veins, has been concentrated in the coal districts. Mr. Bald, Mr. Buddie, and other observers, have long since collected much information in the collieries of the Tyne and Wear; of which, however, we can make only partial use, because the experiments were mostly made on the air, which, for many reasons besides miners' lights and chemical actions, is unlikely to yield accurate ratios, such as are now attainable.

The following are some of Mr. Bald's results, published in the Edinburgh Royal Transactions. The scale is Fahrenheit's.

Whitehaven.—Spring at surface

480 feet

Ratio from surface, 1 for 44 feet.

Workington.—Spring at surface

504 feet

Ratio from surface, 1° for 42 feet.

Percy Main Colliery, Northumberland.—Mean

temperature at surface

900 feet depth

Ratio from surface, 1 for 43 feet.

Jarrow colliery.—Surface assumed

Water at 882 feet

Ratio from surface, 1 for 48 feet.

Killingworth colliery.—Surface assumed

Water at 1200 feet depth

Ratio from surface, 1° for 46 feet.

The near accordance of these results is remarkable. The ratios are all in error by a small quantity, because HO allowance is made for the depth of variable heat.