464 T. W. E. DAVID AND E. F. PITTMAN. 
The above experiments were conducted without any special 
precautions having been taken against convection currents. It is 
impossible, however, that convection currents would obtain to 
such an extent as to materially alter the temperature in a bore- 
hole only four inches in diameter from 2,400 feet to 2,929 feet, 
and five inches in diameter from 2,400 feet to the surface. If 
therefore 97° Fah. be assumed to be the correct temperature of 
the earth’s crust at Port Jackson at a depth of 2,733 feet, (and 
the authors do not think there is likely to be an error of more than 
half a degree in the temperature above quoted), the mean surface 
temperature being 63° Fah., as determined by Mr. H. C. Russell, 
the rate of increase in temperature down to a level of 2,733 feet 
would be 1° Fah. for every eighty feet, after the zone of mean 
temperature, at about thirteen feet below the surface, has been 
passed. The temperature therefore at the depth at which the 
coal seam was struck, 2,917 feet, should be 2°3° Fah. higher 
than that registered ac 2,733 feet, that is 99:3° Fah. 
At the Metropolitan Colliery near Sydney, the rate of increase 
of temperature was found to be approximately 1° Fah. for every 
seventy-eight feet. This result however, can be considered as 
only approximate. Experiments were also made with a view of 
ascertaining whether the water in the deeper portions of the 
harbour may have had any chilling effect locally on that portion 
of the earth’s crust nearest the bore, so as to depress the isogeo- 
therms. The temperature however of the water in the harbour 
at the greatest depths near Cremorne, varying from forty-five feet . 
to sixty-three feet, was uniform at 68° Fah. The experiment 
was made on December 6th, 1893. The abnormally low rate of 
increase in temperature in the earth’s crust downwards at Cre- 
morne cannot therefore be attributed to any chilling effect of the 
water in the harbour. More observations will be needed before 
any definite conclusions can be arrived at. The comparatively 
low temperature at the horizon of the coal seam will obviously 
materially lessen the cost of ventilation, as compared with what 
it would have been, had the rate been 1° for every sixty-three 
