SECT. XXXV. VELOCITY OF COMETS. 371 



orbit for that year ; but after much labour, aided by all the 

 improved methods of calculation, he found Heller's observations 

 so confused, and even erroneous, that he could not determine the 

 curve described by the comet at that time with any precision, 

 and therefore could only predict that the epoch of its return 

 would be some time between 1848 and 1861. Before comets 

 reach the sun they are rarely conspicuous ; but if after passing 

 their perihelion they come near the earth, then they have tails, 

 and become brilliant in consequence of the sun's action upon the 

 matter of which they are formed. Now if the comet in question 

 should pass its perihelion between the months of March and 

 October, it possibly may be as remarkable as ever ; but should 

 it come nearest to the sun in winter, such is the position of its 

 orbit with regard to the earth, that it may pass unnoticed 

 which is very unlikely, as search is being made for it at almost 

 all the observatories in Europe and in the United States. Nearly 

 the whole of its orbit lies below the plane of the ecliptic, and 

 far from the paths of the larger planets, but it extends into space 

 more than twice the distance of Neptune, or nearly six thousand 

 millions of miles from the sun. 



Comets in or near their perihelion move with prodigious velo- 

 city. That of 1680 appears to have gone half round the sun in 

 ten hours and a half, moving at the rate of 880,000 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 sur- 

 face. So near the sun, it would be exposed to a heat 27,500 

 times greater than that received by the earth ; and as the sun's 

 heat is supposed to be in proportion 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 



