234 REV. W. THORP, B.A. — AGRICULTURAL GEOLOaY, ETC. 
dimensions of the fire should have a direct proportion to the area of 
the shaft through which the ascending current of air has to rise ; 
otherwise, the fuel in the grate not being sufficient to rarify the 
whole portion of the air in the flue, the rising current of heated air 
is met by a descending current of cold air, and the circulation is 
impeded. This is a common defect in the construction of house 
chimneys, and the smoke made by the descending current is borne 
back into the room. Treadgold's rule, applicable to these cases, is to 
multiply by seventeen the length of the fireplace in inches, and divide 
by the square root of the height of the chimney in feet, and the 
quotient is the area in inches for the aperture of the chimney. A 
fire at the bottom of a pit having a fireplace twelve feet, or one 
hundred and forty-four inches wide, and a chimney three hundred 
feet high, requires an aperture of only one hundred and forty-four 
inches, or one foot square ; and as the shafts are often eight feet in 
diameter and a hundred yards in length, there should be two or three 
fires to render the ventilation good. In Yorkshire the long method 
of getting coal is adopted, and of this there are two modifications, 
one with single board gates, the air being drawn from one to another 
across the banks where the men are working, and is used where there 
is little fire-damp ; another has double board gates, the air being 
coursed up one and down another, and drawn across the banks where 
the men are working. The great objections to these systems is that 
while the whole air sent down ventilates in one body the whole mine ; 
if a considerable explosion takes place in any part of it nearly the 
whole of the persons usually perish ; the ventilation is stopped, and 
those who are not burnt or killed by the force of the explosion are 
sufi'ocated for the want of pure air. Mr. Buddie, of Newcastle, has 
devised a new scheme for working coal mines in panel work, and 
instead of the working surface being in one extensive area, it is 
divided into quadrangular panels, each containing eight to twelve 
acres, and round each is a solid wall of coal forty to fifty yards thick. 
Through these walls roads and air-courses are driven in order to 
work the coal contained within each, and thus they are connected 
together with the shaft. The advantage of this method is that there 
is less pressure on the coal ; in case of casualty the accident is local- 
