468 Scientific Intelligence. [June 



There are only two other comets which have been observed to ap- 

 pear periodically. These are EnckcVs and Beile's comets ; but these 

 are of very small magnitude. Encke's has appeared two years sooner 

 than can be accounted for by the laws of gravitation. Beile's was 

 discovered in 1826, since which year it has only returned once, but 

 was accelerated one day beyond the results of calculation. These 

 are the two arguments for a resisting medium, and the existence of 

 an ethereal fluid. 



In June 177°* Messier discovered a brilliant comet within the 

 orbit of Jupiter, and Lexell computed it. Previously it was thought 

 that comets moved in a parabola. He inferred, however, that it 

 moved in an ellipse, and that it would return every 5 \ years. It 

 did not return, however, as he had predicted. Laplace, however, 

 shewed in the 2nd chapter and 9th book of his Mecanique Celeste, 

 that in January 1767 it must have entered within the attraction of 

 Jupiter, and was acted upon by that planet so as to give it an ellipse 

 of 5f years for its orbit, that at the end of the first 54 years the sun 

 obscured it, and at the 11 years Jupiter crossed its orbit and entangled 

 it again. This was a splendid triumph of mind over sense, for La- 

 place first gives a general formula for calculating the retardation of 

 a comet with a given track, anterior and subsequent to its appearance, 

 and then merely takes the comet of 1770 as an example for the appli- 

 cation of the rule. 



If the mass of this comet were equal to that of the earth, the at- 

 traction exerted would shorten the year 10,000 seconds; but it has 

 been found that the year is not diminished 2 seconds, and hence, it 

 must be inferior to the mass of the earth. This comet afterwards 

 went in directly among the satellites of Jupiter without disturbing 

 them in the slightest degree : from which circumstance we can 

 scarcely fail to conclude that it was lighter than light. Much dis- 

 cussion has taken place with regard to the nature of the constitution 

 of comets. Sir John Herschel observed a fixed star through the head 

 of a comet. It has been said that the direction of the tail is influ- 

 enced by the sun, but this is incorrect, because some comets have had 

 two tails, one pointing to the sun; and the other from it, while others 

 have had six tails all pointing in various directions, and vacillating, 

 so that comets may be said to wag their tails. Comets do not shew 

 phases like the moon, because they are not solid, but are more like 

 fleecy clouds when the sun shines upon them. M. Arago suggests 

 that the only method of determining whether they are essentially 

 luminous or not must be by the investigation of their phases, and the 

 comparative intensity of their light by means of photometers. It is 

 remarkable, that they grow larger as they recede from the sun. Sir 

 John Herschel has offered two explanations of this: 1. By consider- 

 ing them to consist of particles which have little cohesion, and which 

 move in a variety of orbits, and get closer as they approach the sun : 

 or 2., and this is the most ingenious attempt at the solution of the 

 difficulty ; that when cooled the particles condense as we observe in 

 steam issuing from a kettle, the portion nearest the source of heat 

 being invisible, while at some distance a cloud of vapour is observed. 



Five or six hundred comets have been recorded, but of 1 37 only 

 have the tracks been observed. 



These follow no regular angle of inclination ; neither are the planes 



