16 ON THE HEAT EVOLVED 



pressure. These being the extremes during my experiments, 

 the difference of heat required to maintain the temperature of 

 the air between any two experiments, cannot materially affect 

 their results, and for this variation no correction has been thought 

 necessary. 



Having described the construction of the interior room, and 

 its apparatus, it remains to describe the exterior room, which 

 has a capacity of 860 cubic feet, after deducting 542 feet for 

 the space occupied by the interior room, and the materials of 

 which it is composed. This room has a southern aspect, and 

 is defended from the west winds by a building projecting be- 

 yond it ten feet south. It has one window, with blinds on the 

 outside, to exclude, when necessary, the rays of the sun ; the 

 east and south walls are of brick, and are ten inches in thick- 

 ness ; the remaining two are partitions of lath and plaster, four 

 and a half inches thick, and separate between a passage on the 

 west, and a room on the north. The chimney is in the east 

 wall. A small stove is placed in this room, the pipe of which 

 passes through the fire-board. A mercurial thermometer, to 

 measure the temperature of the air, is placed in a convenient 

 situation, on a line with those in the interior room, and on a 

 table an accurate balance is suspended, to weigh the articles 

 which are to be subjected to experiment. 



The plan of the experiments will next be described. 



Equal quantities are taken of each article by weight, pre- 

 viously made absolutely dry ; by which is to be understood, 

 that state of deprivation of moisture manifested when no di- 

 minution in weight can be effected by the heat of a stove at 

 250 c of Fahrenheit. 



It is required to determine the period of time which the 

 combustion of each article will maintain the temperature of 

 the interior room 10° higher than the exterior; and the time 

 that the interior room is thus maintained by any article, gives 

 its true relative heat, when compared with the time which any 

 other article has maintained the room at the same difference of 

 temperature. As the temperature of the air in both rooms is 

 supposed to remain stationary, the increments and decrements 



