190 WILLIAMSON ON COMETS. 



cannot enjoy the same degree of heat, unless the weight of their atmos- 

 phere be increased according to their several distances from the sun. 



3. We are now to consider whether the several planets are, in fact, 

 provided with an atmosphere ; and whether such atmosphere be propor- 

 tioned to their several distances from the sun. These inquiries appear 

 the more advisable, because we should with little propriety attempt to 

 prove that comets are habitable, unless we could prove that the planets, 

 with which we are better acquainted, are habitable. We are sufficiently 

 acquainted with the atmosphere of this globe. We know that its 

 weight upon a square foot of the surface is equal to two thousand one 

 hundred and sixty pounds, and that its height does not much exceed 

 fifty miles; at least its density, above that height, is not sufficient to 

 refract the rays of light. Of the other planets we can only say with 

 certainty, that some of them have an atmosphere, but the presumption 

 is strong that they are all provided with the same covering. We are 

 certain that Venus has an atmosphere ; for in the year 1 769, observing 

 the transit of Venus over the sun, at the Observatory in Philadelphia, 

 her external contact was somewhat obscured, as we afterwards pre- 

 sumed, by her atmosphere; but when half of the planet was on the sun, 

 we discovered a clear circular light round the other half that was off 

 the sun. That light was evidently the rays of the sun refracted by the 

 atmosphere of the planet. Other people in different parts of the world, 

 on the same day, noted similar proofs of the atmosphere of Venus. If 

 Mercury has an atmosphere, considering his very small distance from 

 the sun, his atmosphere should be very thin. It happened that Mer- 

 cury passed over the sun, on the 9th of November in the same year; and 

 the same persons were appointed to observe his transit. On that occa- 

 sion the external contact of Mercury with the sun's limb appeared sud- 

 den, without any previous obscuration; nor did either of us discover 



