

218 Observations on the Comet of Halley. 



incr as we do but imperfectly the masses of the planets, and possibly 



their number also, it is not strange that astronomers should have 



been led to different results, in estimatii 



turbing influences. 



A highly interesting question here arises, Is the comet gradually 



wasting away, so that there is reason to apprehend that its matter 



may ultimately become entirely dissipated in space ? 



According to the usually received theories, it seems necessary to 

 suppose that much of the matter which is thrown off to form the 

 comet's train, must be forever dissipated, so that a gradual diminu- 

 tion of the comet's mass appears inevitable ; yet observations shew 

 that in the case of Halley's comet, at least, this diminution has been 

 very gradual, if indeed it has been perceptible. It is certain, that 

 the comet has not, at every return, presented the same splendid ap- 

 pearance, yet perhaps these various appearances may be accounted 

 for, from the different positions of the comet with reference to the 

 earth and sun, without admitting an actual diminution of its mass. At 

 its last return, it was visible to the naked eye for five or six weeks, 

 before its perihelion passage, and for several evenings exhibited a 

 train from 10° to 12° in length. In 1759, the comet attracted lit- 

 tle attention in Europe, except from astronomers. Most observers 

 were doubtful whether it had any train. This however, is very ea- 

 sily explained. The comet was at too great a distance to be seen 

 by the naked eye, until after the perihelion passage. In April, 

 1759, it approached somewhat nearer to the earth, than it did at its 

 last visit ; yet it had then passed its descending node ; had a great 

 southern declination, so that its meridian altitude at London was 

 only about 10° or 15°, was consequently almost entirely lost in the 

 mists of the horizon, and was still further obscured by the presence 

 .of the moon. In the southern hemisphere, however, where the 

 comet had a great elevation, it shewed a conspicuous train, varying, 

 according to La Caille, from 10° to 47°. The appearances in 

 1682, were not greatly different from those in 1835. Flamsteed's 

 observations are the following: Aug. 19, train about 5° in length 

 Aug. 22, train 10° in length— Aug. 30, train scarcely 2° in length 



Sept. 1, scarcely any train could be seen by the naked eye. 

 Hevelius at Dantzic observes: Comet first seen Aucr. 25. The 

 head was rather brighter and larger, than that of the year 1681, but 

 it had a much shorter train. At first, it was about 12° long; after- 

 wards rather shorter, and sometimes longer, as far as to 15° or 16°; 



