348 Mr Milne'*s Prize Essay mi Comets. 



perihelion distances and periods of revolution are not the same ; a con- 

 dition confirmed by the examination of several, that have been the most 

 attentively observed. The second Comet of 1811 had a nucleus, which, 

 according to the continental astronomers, amounted to 570 miles * ; 

 while its tail was 500,000 miles in length. The Comet of 1807 pos- 

 sessed a nucleus of less size, but a tail of greater brilliancy ; the diame- 

 ter of the one being only 538 miles, the length of the other 9,000,000. 

 The first Comet of 1811, which, from its splendid appearance, has been 

 termed the great Comet of 1811, was observed to have a smaller nu- 

 cleus ; but, on the other hand, its envelope and tail were far more ex- 

 tensive : the diameter of its nucleus was 428 miles, and its tail stretched 

 out no less than 132,000,000 of miles. The first of these three comets, 

 then, according to Herschel's theory, must have been subjected in a 

 much gi'eater degree to the consolidating influence of the sun's heat 

 than either of the other two, seeing that it had the largest nucleus, and 

 the least quantity of nebulous matter : and the like result ought to be 

 indicated with respect to all the three comets, on a comparison of their 

 respective periods and perihelion distances. The periodical revolution 

 of the great Comet of 1811 is found to be 3383 years, and it approaches 

 1.55 nearer the sun at its perihelion, than the other Comet of 1811 : 

 the product of these two numbers is 5243. The periodical revolution 

 of the Comet of 1807 is 1713 years, and its perihelion distance is 2.46 

 times less than that of the second Comet of 1811 : the product of these 

 two numbers is 4213. The periodical revolution of the second Comet 

 of 1811, whose perihelion distance we have taken equal to 1 as the 

 standard of comparison, is 875 years. These numbers, then, 5243, 

 4213, 875, representing inversely the result of the sun's long continued 

 action upon the nebulous matter of the three comets, correspond very 

 nearly with the relative magnitudes of their nuclei, as indicated by ob- 

 servation ; and hence the confirmation of Herschel's theory is complete. 

 If this comparative view of comets be verified by more extended obser- 

 vations, it will serve to give some insight into the origin and aiTange- 

 ment of these bodies, and inform us of the true place which they occu- 

 py in the planetary system. Nor will it be the least important result 

 of the establishment of this theory, that it will enable astronomers to 

 arrange comets according to the various stages of maturity at which, in 



• Brande's Astron. 11. 31. I may here again advert to the difference in 

 the measurements of this comet, made by Shroter and Herschel. If we as- 

 *8ume the measurement given by the latter, it becomes even more favourable 

 to the theory submitted in the text. 2 



