19^ -^n Essay on a Proper Uj in Light 



yerely felt by them, when they were at a distance from tlie sun, 

 if they had no other supply than the scanty portion which, in 

 that situation, he would be able to afford them. 



It will be acknowledged by every, one, that although the sun is 

 the source of heat, we are chiefly indebted to the atmosphere for 

 all the warmth that we enjoy. Without an atmosphere, or with 

 a very thin one, we should be insensible to the sun's heat, as 

 people never feel any warmth from a fire till the air in the room 

 is well heated by it. The denser the atmosphere is, that is, the 

 more substance it contains, the more heat it receives and the 

 longer it retains it; or the top of a high mountain would not be 

 covered with snow when the foot of it was scorched up with heat ; 

 and as the weight of atmosphere above presses the particles be- 

 low into a smaller space, the density of the atmosphere is always 

 proportioned to its depth or quantity. 



In whatever part of the system therefore the power of the sun's 

 heat may be the greatest, whether in Mercury or .Jupiter, and 

 one seems as probable as the other*, a greater or a less quantity 

 of atmosphere would give to all the planets an equal tempera- 

 ture, and none of them would suffer greater extremes of heat 

 and cold than we do in our summer and winter. 



Now, though a greater quantity of atmosphere would give to a 

 distant planet all the heat that a nearer one enjoyed, this rule 

 will not answer for the comets; because the same quantity of at- 

 mosphere which would be necessary to give to their inhabitants 

 a sufficient quantity of heat, for the purposes of existence when 

 they were furthest from the sun, would destroy them when they 

 were near. The only way I can imagine to remedy this incon- 

 venience is to give to the comets a moderate heat of their own, 



with 



* If the sun be a heated body, like red hot iron or a burning coal, it is 

 probable that his greatest heat is immediately upon his surface ; but if he 

 be an incombustible body sending out rays of light and heat ; (and if it were 

 otherwise, we ought to see symptoms of his decay ;) it is then most likely 

 that hia greatest heat is at a considerable distance beyond his surface, even 

 though the rays of heat, like the rays of light, lose a great deal of their sub- 

 stance, or intensity, before they arrive at that point. If we suppose a person 

 to be standing on the surface of tiie sun, all the rays that could possibly 

 strike upon him, vould be included within his visible horizon (a circumfe- 

 rence of not more than twenty miles) : if afterwards he was lifted up till his 

 eye took in the whole of the sun's hemisphere (a circumference of more than 

 two millions of miles), he would have two hundred thousand times more rays 

 strike upon him than when he was on the sun's surface ; and therefore the 

 rays must be two hundred thousand times less heated here than when they 

 flit issued from the sun, which is not at all likely, before he could suffer 

 any diminution of tbeir heat. Even beyond that point, every time he doubled 

 his distance from the sun, and at first this distance would be very short, he 

 would have twice the quantity of rays striking upon him ; and until the sun's 

 angle hecame very ftnc, that iv, until he had a great way to travel '.n order 



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