118 RECORD OF SCIENCE FOR 1886. 



the instrumental constants determined by using the ej'e-piece in the 

 usual M^ay. 



COMETS, 



Professor Bredichin in continuing his researches upon the mathe- 

 matical theory of comets has re-determined the repulsive forces which 

 produce the tails of different types. Making use of some forty comets in 

 his discussion, he has found for tails of type i, a mean value, 1 — // = 14; 

 but the comet of 1811, by far the most favorable for the determination 

 of the repulsive force of this type, gave 17.5, and this represents quite 

 well the tails of other comets. In this type the initial velocity g varies 

 from 0.1 to 0.34, the mean being 0.23 (0.1=1.9 miles per second, about). 

 In type ii the forces vary from 0.5 to 2.2, and the initial velocities from 

 0.03 to 0.07, mean 0.05. For the axis of the tail 1 — yu=l.l. In type 

 III the repulsive forces lie between 0.1 and 0.3, and the velocities be- 

 tween 0.01 and 0.02. 



Dr. Holetschek's investigation upon the conditions of visibility of a 

 comet have been followed up by Dr. W. Meyer, who finds that if the 

 great comets of 1843, 1880, and 1882 had reached perihelion in May 

 they would have escaped unobserved. The orbit of the comet seen 

 during the total eclipse of May 16, 1882, must have been very much 

 like that of the comet which appeared four months later (1882 II) ; it 

 seems, indeed, that the observed position can be represented to half a 

 degree by the elements of the September comet, merely changing the 

 time of perihelion of the latter and fixing it for the 17th of M.Q.J. The 

 ephemeris computed by Dr. Meyer with these elements shows very 

 plainly why the Sohag comet could not be found after the eclipse, or 

 had not been detected before; it was too faint when in a position favor- 

 able for observation. The comet is probably one of a regular stream of 

 comets with small perihelion distance, such as 1843 I, 1880 I, 1882 II. 

 If the orl)its of the comets of 1843 and 1880 were sufficiently alike in 

 other res[)ects, the failure in repeated returns would be no objection to 

 their identity, for if the returns have taken place in the month of May, 

 the comet must have been invisible. A revolution in thirty-seven 

 years is hardly to be reconciled, however, with the observations of 

 1843, and for the great comet of 1882 Frisby has found a period of 

 seven hundred and ninety-four years. 



Mr. Monck, in the Observatory for August and September, brings 

 out some interesting statistics in support of his view that there exists 

 a sort of " companionship " among comets — that is, cases in which the 

 elements show a striking similarity; but it is improbable that the 

 bodies are identical. Several of the comets of short period exhibit a 

 family likeness which can hardly be attributed to their capture by 

 Jupiter unless they previouslj" formed members of a system. The ques- 

 tion derives further interest from its bearing upon meteoric showers, 

 for, if a family of comets can be supposed to be accompanied by a 



