VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 755 



this purpose, an assistant thermometer, which should always remain in the nearest 

 convenient place to the apparatus, will be of use, to point out the time when the 

 experiment may be begun ; for this ought not to be done till the thermometers to 

 be used agree with the standard. In order not to lose time after an experiment, the 

 apparatus may be taken into a cool room, or current of air, till the thermometers 

 it contains are rather lower than the standard ; after 

 which, being brought to the required situation, they 

 will soon be fit for action. All these precautions having 

 been taken, I began the experiment by first writing 

 down the degrees of the thermometers ; then, opening 

 the cover at the time that a clock or watch showing 

 seconds came to a full minute, I continued to write 

 down the state of the thermometers for not less than 5 minutes, 

 as annexed. Here the sun communicated, in 5 minutes, 6° of heat to the thermo- 

 meter N° 5, which was openly exposed to its action ; while, in the same time, N° 1 

 received only 4± degrees by rays transmitted through the bluish-white glass : then, 

 as 6 : 4-1 :: 1 : .750. This shows plainly, that only $ of the incident heat were 

 transmitted, and therefore that 4- of it was intercepted by the glass. 



I shall here, as well as in the following experiments, point out the difference 

 between heat and light, in order, as has been mentioned before, to lead to an elu- 

 cidation of our last discussion. To effect this therefore, I have ascertained, with all 

 the accuracy the subject will admit of, the quantity of light transmitted through 

 such glasses as I have used; but as it would here interrupt the order of our subject, 

 I have joined, at the end of this paper, a table, with a short account of the method 

 that has been used in making it, wherein the quantity of light transmitted is set 

 down ; and to this table I shall now refer. To render this comparative view more 

 clear, we may suppose always 1000 rays of heat to come from the object : then 750 

 being transmitted, it follows that the bluish-white glass used in our experiment 

 stops 250 of them ; and, by the table at the end of this paper, it stops 86 rays of 

 light ; the number of them coming from the object also being put equal to 1000. 

 It should be remarked, that when I compare the interception of solar heat with that 

 of the light of a candle, it must not be understood that I take terrestrial to be the 

 same as solar light ; but not having at present an opportunity of providing a similar 

 table for the latter, I am obliged to use the former, on a supposition that the quan- 

 tity stopped by glasses may not be very different. 



N° 5. N° l. 

 Eocper. 25. I took a piece of flint glass, about 2-1- tenths of an sun. Flint glass. 



inch thick, and fastened it over one of the holes of the trans- J?f 6 9£ 



. 71$ 71 



mitting apparatus. Here the heat-making rays gave, in 5 minutes, 724 72^ 



54- degrees to the thermometer N° 5; and, by transmission through £H 73% 



the flint glass, 5° to N° 1 . Then, proceeding as before, we have 75 | 74 | 



5d 2 



