on the terrestrial Rays that occasion Heat. 4,6*3 



a hole in each, \ inch in diameter, through which the heat of the 

 candle passes to the two thermometers, which are to be placed 

 in opposite directions, one on each sjde of the table. Care must 

 .be taken to place them exactly at the same distance from the 

 centre of the flame, as otherwise they will not receive equal 

 quantities of heat. The scales, and their supports also, must be 

 so kept out of the way of heat coming from the candle, that they 

 may not scatter it back on the balls, but suffer all that is not 

 intercepted by them to pass freely forwards in the box, and 

 downwards, through openings cut in the bottom. Before the 

 transmitting holes, between the two wooden screens, must be 

 two covers of the same material, close to the openings ; * and it 

 will be necessary to join these covers at the side, by a common 

 handle, that they may be removed together, without disturbing 

 any part of the apparatus, when the experiment is to begin. 



The glasses are to be put before the thermometer, close to 

 the transmitting hole, by placing them on a small support 

 below, while the upper part is held close to the screen by a 

 light plummet, suspended by a thread which is fastened on one 

 side, and passes over the glass, to a hook on the other side. 



In making experiments, many attentions are necessary, such 

 as, keeping the candle exactly to a certain height, that the 

 brightest part of the flame may be just in the centre of the two 

 transmitting holes : that the wick may be always straight, and 

 not, by bending, approach nearer to one thermometer than 

 to the other : that the wax-cup of the candle be kept clean, 

 and never suffered to run Over, &c. 



Before, and now and t|ien between, the observations also, the 

 thermometers must be tried a few degrees, that it may be seen 



* See Fig. 2. 



3O2 Jl 



