422 HISTORY OF SCIENCE. 



was subject to variation. In 1852, when the two comets returned, the 

 space between them was found to have increased to 1,250,000 miles. 

 The next perihelion passage of these comets would occur in 1859, 

 but in a position unfavourable for observation. They were not seen 

 in 1859, and although they have been expected and sought for at the 

 periodic times, viz., in 1866 and 1872, they have not been seen. 



The times of several other comets of short periods have also been 

 determined. Their orbits, like those of Encke's and Biela's comets, all 

 lie within the orbit of Jupiter, and they are liable to great perturba- 

 tions caused by the attraction of that huge planet. The periods of a 

 few other comets, varying from sixty-seven to seventy-seven years, have 

 also been determined. The best known of these is the comet named 

 after Halley, because this eminent astronomer, having made elaborate 

 calculations from the records of remarkable comets which had appeared 

 from time to time, with a view of discovering whether the comet he 

 himself observed in 1682 had previously appeared in the same path, 

 thought probably that it and certain comets seen in 1531 and 1607 

 were the same body. There were, however, discrepancies in the cir- 

 cumstances of the successive periods not being equal, and the inclina- 

 tion of the comet's orbit to the ecliptic being somewhat different in 

 each case. Halley attributed these changes to the perturbing action of 

 Jupiter, and by calculation he found that between 1607 and 1682 his 

 comet must have passed so near to the planet that its motion would 

 be accelerated and its period shortened. He predicted its return in 

 1758. The last return of Halley's comet was in 1835, when it passed 

 through perihelion within four days of the time which had been as- 

 signed. The orbit of this comet was studied by Damoiseau in 1817, 

 and by Ponte'coulant in 1829. Records of its successive appearances 

 have been recognized by Hind in the accounts of great comets from 

 B.C. ii downwards. The orbits of a great number of other comets 

 have been calculated from the records of observations, but with little 

 of interest in the results. It may here be mentioned that the method 

 of deducing the most probable true value from a number of observa- 

 tions liable to be affected by errors, called the method of least squares, 

 was first applied to calculations for comets, and has always since been 

 in use for such calculations. 



Several remarkable comets have been observed in the present cen- 

 tury. In the autumn of 1811 a great comet was conspicuous in the 

 heavens. Its tail was 25 in length, and about 6 in breadth. Its 

 orbit was calculated by a German astronomer, and was pronounced to 

 be elliptical, and to the comet the period of revolution was assigned 

 of 3,065 years. A very brilliant comet, with a tail 40 in length, was 

 seen in February and March, 1843. The orbit of this comet had an 

 unusually small perihelion distance, for it approached to within 538,000 

 miles of the sun ; and the comet swept through the part of its course 

 near the sun with inconceivable velocity. In one day it passed through 



