504 ON THE VALUE OF THE MECHANICAL EQUIVALENT OF HEAT. 
regulator exceeding 1 centim. In order to diminish the effect of changes in the 
atmospheric pressure, some air was allowed to remain in the horizontal tube. As the 
column of mercury communicating with the regulator was about 800 mi llim s, in 
height, this residual air was under a pressure of two atmospheres, and, its volume 
being very small, and the diameter of the mercury tube large, a small movement 
sufficed to compensate changes in atmospheric pressure. Thus, an alteration of 
20 millims. in the barometer produced a change of only a fraction of a degree in the 
resulting laboratory temperature. The gas, after passing through the regulator, 
communicated with a large Doulton ware stove, in which small pilot lights were 
always kept burning by means of an independent supply. An entrance and an exit 
tube, closed by glass taps, were so fixed in the vertical tube containing the mercury 
that, by their means, the instrument could be adjusted. 
The action of the apparatus is, in many ways, interesting. On a day when the gas 
would be burning, if the room was unoccupied, the presence of one person in the room 
would diminish the supply, whereas the j^resence of two or three would cause it to be 
entirely cut off. We were surprised to learn, by this means, the amount of heat 
communicated to a room by the presence of a human being. 
