400 FALL OF COMETS TO THE SUN. [SECT, xxxvi. 



miles an hour. If its enormous centrifugal force had ceased 

 when passing its perihelion, it would have fallen to the 

 sun in about three minutes, as it was then less than 147,000 

 miles from his surface. So near the sun, it would be ex- 

 posed to a heat 27,500 times greater than that received by 

 the earth ; and, as the sun's heat is supposed to be in pro- 

 portion to the intensity of his light, it is probable that a 

 degree of heat so intense would be sufficient to convert 

 into vapour every terrestrial substance with which we 

 are acquainted. At the perihelion distance the sun's 

 diameter would be seen from the comet under an angle of 

 73, so that the sun, viewed from the comet, would nearly 

 cover the whole extent of the heavens from the horizon to 

 the zenith. As this comet is presumed to have a period 

 of 575 years, the major axis of its orbit must be so great, 

 that at the aphelion the sun's diameter would only subtend 

 an angle of about fourteen seconds, which is not so great 

 by half as the diameter of Mars appears to us when in 

 opposition. The sun would consequently impart no heat, so 

 that the comet would then be exposed to the temperature of 

 the ethereal regions, which is 58 below the zero point of 

 Fahrenheit. A body of such tenuity as the comet, moving 

 with such velocity, must have met with great resistance 

 from the dense atmosphere of the sun, while passing so 

 near his surface at its perihelion. The centrifugal force 

 must consequently have been diminished, and the sun's 

 attraction proportionally augmented, so that it must have 

 come nearer to the sun in 1680 than in its preceding revo- 

 lution, and would subsequently describe a smaller orbit. 

 As this diminution of its orbit will be repeated at each re- 

 volution, the comet will infallibly end by falling on the 

 surface of the sun, unless its course be changed by the dis- 

 turbing influence of some large body in the unknown ex- 

 panse of creation. Our ignorance of the actual density of 

 the sun's atmosphere, of the density of the comet, and of 



