on the terrestrial Rays that occasion Heat. 305 



Here, in three minutes, the thermometer fell 16 degrees. I 

 opened the mirror again. 



No. 2. 



In the Focus of invisible Heat. 



5' 83 



6 88 



Here, in two minutes, the thermometer rose 24 degrees. I 

 covered the mirror once more. 



7' 69 



And, in one minute, the thermometer fell 19 degrees. Now, 



by this alternate rising and falling of the thermometer, three 

 points are clearly ascertained. The first is, that there are invisible 

 rays of the sun. The second, that these rays are not only re- 

 flexible, in the manner which has been proved in the foregoing 

 experiment, but that, by the strict laws of reflection, they are 

 capable of being condensed. And, in the third place, that by 

 condensation, their heating power is proportionally increased ; 

 for, under the circumstances of the experiment, we find that it 

 extended so far as to be able to raise the thermometer, in two 

 minutes, no less than 24 degrees. 



gth Experiment. Reflection of invisible culinary Heat. 



I planted my little steel mirror upon a small board ;* and at a 

 proper distance opposite to it , I erected a slip of deal, ^ inch 

 thick, and 1 inch broad, in a horizontal direction, so as to be 

 of an equal height, in the middle of its thickness, with the 

 centre of the mirror. Against the side, facing the mirror, were 

 fixed the two thermometers No. 2 and No. 3, with their balls - 

 within half an inch of each other, and the scales turned the 

 opposite way. A little of the wood was cut out of the slip, to 



* 6ee Plate XIII. Fig. I. 



Rr 2 



