400 



is rarefied and made specifically lighter than the descending 

 current, whether by the men and animals in the pit, the natural 

 temperature of the mine itself, or by a fire placed in the 

 upcast shaft. And the efiiciency of the ventilation of the 

 mine will, ceteris paribus, be in proportion to the heat of the 

 ascending current in the upcast pit. By the experiments of 

 Gay Lussac, 100 parts of atmospheric air, heated from 32° 

 to 212°, expand 137.5 parts; or 180° of heat expands it 

 about one-third. The rate of expansion or rarefaction is 

 not exactly equal for equal increments of heat, but on an 

 average is of its bulk for each degree of Fahrenheit. 

 If heat be communicated to a particle of air, a change with 

 respect to that particle takes place in the following manner : 

 it becomes expanded and increased in bulk but not in weight, 

 and in consequence rises from among the other particles, and 

 ascends upwards. The moment its gravity becomes altered, 

 and in consequence is rendered specifically lighter than the 

 surrounding particles, it ascends and passes upwards through 

 the surrounding particles. And the moment any particle or 

 portions of air have moved upwards from what heated it, 

 their places are taken by other portions coming out of the 

 interior of the coal mine; which last portions, undergoing 

 the same change, ascend in like manner, having their places 

 taken by other particles ; and this alteration continues with a 

 rapidity in proportion to the heat communicated, whether the 

 heat be that of human bodies, stoves, fires, &c. If air be 

 heated to 572°, it will be half the weight of that at 60% and 

 the downward pressure equal to 7J lbs. per inch, into the 

 downcast shaft. 



It becomes, therefore, an object of the first importance to 

 obtain a rapid and warm current of air in the upcast shaft, 

 and for this purpose a fire is placed in all mines with extensive 

 workings at the bottom of the upcast pit ; and here occurs a 

 diificulty not usually acknowledged by the viewers and over- 



