452 Dr. Herschel's Experiments on the solar, and 



pass. It will be sufficient to say, that they have all been made 

 on the same plan as the two which have been given. The 

 observations were always continued for at least five minutes ; 

 and, by examining the ratios of the numbers given by the ther- 

 mometers in all that time, it may be seen that, setting aside little 

 irregularities, there is a greater stoppage at first than towards 

 the end ; but; as it would not be safe to take a shorter exposure 

 than five minutes, on account of the small quantity of heat 

 transmitted by some glasses, I have fixed upon that interval as 

 sufficiently accurate for giving a true comparative view. The 

 experiments therefore may now stand abridged as follows. 



&6tb Experiment. 



I took a piece of highly polished crown glass, of a greenish 

 colour, and, cutting it into several parts, examined the transmit- 

 ting power of one of them, reserving the other pieces for some 

 other experiments that will be mentioned hereafter. 



Sun. Greenish crown glass. 



o' 66$ 66± 



5 73 7ii--- 6 i : 5 = >74 1 



This glass therefore stops 259 rays of heat, and 203 of light. 



27^ Experiment. 



I cut likewise a piece of coach glass into several parts, and 

 tried one of them, reserving also the other pieces for future 

 experiments. 



Sun. Coach glass. 



o' 68$ 68f 



5 75i 74>i ■ • • 7 : Sk = >7*6 



It stops 214 rays of heat, and 168 of light. 



