The volume and velocity of the air currents vary
greatly. In one place a circulation of 5000 cubic feet
per minute is recorded. In another pit the circulation
was 100,000 cubic feet per minute, while in the up-



186



ON UNDERGEOUND TEMPERATURES



[VI



cast shaft, 7|- feet in diameter, of another pit, 150,000
cubic feet of air passed per minute.
At the Hetton Colliery (No. 116), Durham, the
dependence of the temperature of the strata upon dis-
tance from the shaft, in connexion with ventilation, is
well shown in a table by Mr. Wood, of which the fol-
lowing is an abstract.
Surface temperature at date = 36°. Holes in coal
3 feet deep.
Depth from
surface.
Distance
from shaft.
Cubic feet
of air per
minute.
Temp,
of coal.
Temp.
in current

of air.
Eate of

increase in

depth for

I'F.
feet.
1,100
1,270

■ .i.seo

1,354
1,400
1,395


yards.
312
915
1,640
3,664
3,550
4,332


104,000

60,000

18,000

3,200

4,972

5,000


60"

63

66

68

701

71


50°

m

62
68i
72
73


feet.
100
91
80
67
64
63i



In this case we have four observations made nearly
at the same depth, and yet showing a rate of increase
w^ith depth varying from 80 feet to 63| feet for 1°
Fahr.
In the Seaham (Durham) Colliery the circulation of
air is very large, — 193,346 cubic feet per minute. At
a depth of 1524 feet and 1 642 yards distant from shaft,
the temperature of the air was found to be 61° ; at 2T26
yards distant and at the same depth it was 67° ; and
at the face of the coal 82°. The temperature of the
return air = 76^°.
