492 SCIENTIFIC RECREATIONS. 



The calculation for its actual appearance was made by Claivant, and the 

 expected visitor passed the perihelion in March 1759. This comet, on its 

 appearance at Constantinople, is said to have caused much consternation, 

 and Christians regarded it as a "sign," for the Turks had just then captured 

 Constantinople, and were threatening Europe. Pious people included it in 

 their supplications for deliverance from their most dreaded enemies, and 

 " Lord, save us from the Turks, the devil, and the comet," was a common 

 prayer. 



There have been several very beautiful comets. Encke's, Coggia's, etc., 

 and the comet of 1858 (Donatis) must be in the recollection of middle-aged 

 readers. Others came in 1861 and 1874. In 1881 two comets appeared. 

 Some comets of antiquity were very remarkable, and are reputed to have 

 equalled the sun in magnitude. One tail is usually supposed to be the 

 distinguishing mark of a comet, but in 1774 one appeared with six tails, 



Fig. 538. Great comet of 1811. 



arranged something like a fan. Sometimes the tail is separated from the 

 head. Of the actual consistency of comets we cannot give any lengthened 

 details. They apparently consist of elements similar to the meteors namely, 

 of solid masses, and have been supposed to be aggregations of meteors. 

 Some appear at regular intervals, and their approach can be determined 

 with accuracy. Of course we only see those which are attracted by the sun, 

 or those which revolve in the solar system. There must be thousands of 

 other comets which we never see at all. 



The diagram (fig. 540 in the next page) represents a portion of the path 

 of the comet of 1680. This visitor pursued its course for two months at 

 a velocity of 800,000 miles an hour. The tail was estimated to extend 

 123,000,000 of miles, and a length of 60,000,000 of miles was emitted in 

 two days. When this great comet was approaching the sun, or its perihe- 

 lion, as such approach is termed, three minutes more would have seen it 

 rush into the orb had its enormous pace been slackened, but as it was pro- 



