14 ON THE HEAT EVOLVED 



apparatus; its peculiar applicability to my experiments not 

 having previously occurred to me. 



The differential thermometer used in my experiments, indi- 

 cated £0°, to 1° of the mercurial thermometers, and, as one of 

 the bulbs is situated in the interior room, it can only be ope- 

 rated upon by the temperature of that room ; the other bulb 

 being in the exterior room, can only be operated upon by the 

 temperature of the latter room ; consequently, any change of 

 temperature in either will be shown on the scale, the instru- 

 ment having been adjusted with great care, so that the top of 

 the tinged liquor will stand at 50°, when there is a difference 

 of 10° between the mercurial thermometers placed in the two 

 rooms ; and from its superior sensibility in detecting incipient 

 changes, the differential thermometer may almost be said to 

 possess the power of divination, whereby the operator receives 

 timely notice to avoid any essential error. 



Fig. 5, is a tin supply pipe, two inches in diameter. This 

 passes through the floor in a perpendicular direction, and has 

 an elbow joint opening towards the stove. It has a valve to 

 regulate the quantity of air found necessary to be admitted into 

 the room for the purposes of respiration, and to support the 

 combustion in the stove. This valve, when once adjusted, re- 

 mained the same through all the experiments. Whether the 

 precise quantity of air necessary for the respiration of the 

 operator, and to support the combustion, is admitted by this 

 pipe, or an excess, its temperature being the same, and the 

 stove being supposed always to be supplied with air at the 

 temperature of the interior room, and to require about the same 

 quantity during any given period of two or more experiments, 

 the air admitted being also of equal volume, its velocity will 

 be the same under all changes of barometric pressure ; conse- 

 quently, the reduction of the temperature of the air in the room 

 may be supposed to be the same during the time required to 

 perform each experiment, with the exception of an immaterial 

 variation in its specific heat, to be hereafter noticed ; and, 

 the results of the experiments cannot be affected by the admis- 

 sion of an excess of air, they being, as before stated, founded 

 on the comparative, and not the positive quantity of heat evolved. 



