318 



APPENDIX. 



sun's rays ; as a piece of iron will be more heated 

 than a piece of wood. 



We have seen, then, that the solar rays are not 

 hot in themselves, that they produce Heat only 

 when they are bent or destroyed, that therefore 

 they do not heat transparent bodies, nor do they 

 heat in passing through them, but only at tha 

 entrance and passing out again. 



It is evident^ since the rays of the sun have a 

 power of producing Heat, that they will heat a 

 .body more, the greater the quantity is that falls 

 on it. From this principle, combined with some 

 particular circumstances, arises the different Heat 

 and Cold of the seasons, and of the different parts 

 of the earth. For instance, the Heat about the 

 equator has been known to arise to -f- 1 10 : and 

 it is said, that the Cold in Siberia has been as low 

 as + 160; but the accuracy of this may be 

 doubted, for I do not see how so great a degree of 

 Cold can be precisely measured. 



The Heat produced by the solar rays is increased 

 also by reflection of them : if they be frequently 

 reflected, a greater Heat is produced than even if 

 they be all destroyed. Thus we find that, if we re- 

 ceive them into a box so constructed as to reflect 

 them frequently from side to side, more Heat will 

 be produced than in a box made black, so as to 

 destroy them almost all. Hence, in vallies, even 

 in temperate climates, where the rays are reflected 

 frequently, very great Heal is also produced. 



The different distances of the planets from the 



sun. 



