1 12 On the Formation of the Tails of Comets. 



part which gave the most light. This result may be tested by 

 a comparison with the results of other observations. Thus, the 

 breadth of the tail at its extremity, on the same evening, was 

 estimated to be a little less than 3°. Taking it at 3°, I find the 

 actual breadth to have been 7,040,000 miles. Now according to 

 Mr. Caldecott, of the Royal Observatory at Trevandrum, the neb- 

 ulosity several days after the perihelion passage was 20,000 miles 

 in diameter; and Prof. Bartlett, of the U. S. Military Academy, 

 found its diameter to be nearly 40,000 miles, on the 29th of 

 March. Taking either one of these numbers for the diameter on 

 the 11th of March, we find the point of the tail at which the 

 breadth was one third of that of the end, to be very nearly at the 

 distance of one third of the length of the tail, from the head — 

 within ? j 7 of this length if the first number be used. The incli- 

 nation of the sides of the tail, on the 3d of March, as determined 

 by Piazzi Smyth, Esq., of the Royal Observatory at the Cape of 

 Good Hope, and corrected for the obliquity under which the an- 

 gle was viewed, furnishes a similar result for that date. It would 

 seem therefore that the variation of breadth must have been at 

 least as great as the estimate on the 11th of March would make 

 it. This being allowed, I remark that this increase of breadth 

 in the proportion of 1 to 3, if we suppose the quantity of matter 

 to have been the same in each section, ought to have made the 

 light at the extremity three times less, provided there had been 

 no variation in the inclination of the line of sight to the line of 

 the tail. The variation in this angle which actually obtained, 

 reduces the ratio just given to 2.44. This supposes howeyer that 

 the quantity of light emitted, varies only with the density of the 

 cometic matter. But, if we adopt the theory most in vogue, that 

 comets receive their light from the sun, there must be an increase 

 of this ratio in the proportion of 4 to 1. We thus obtain as the 

 final result 9.76. Now we have seen that the brightest portion 

 of the tail gave about as much light as a star of the third magni- 

 tude. To compare this intensity of light with that at the extrem- 

 ity, it is only necessary to observe that, the moon being little 

 past her first quarter on the evening of the 11th of March, stars 

 of the fifth magnitude were scarcely, if at all discernible, and 

 then to employ the experimental comparisons of the light of the 

 stars, obtained by Sir William Herschel. In this way we find 

 the ratio of intensities to have been as 1 to 4. Some allowance 



