310 Astronomy. [Lecture 19. 



exterior the beams, or hair, in Latin coma, whence 

 the name comet, or hairy star. 



It happens commonly, that a comet is accom- 

 panied by a train of light, sometimes very long, 

 as at L, and always directed to that part of the 

 heavens which is directly, or nearly, opposite to 

 the sun; this is called the tail of the comet. 

 Newton attributes the rise and the direction of 

 the tails of comets to the levity of certain par- 

 ticles, which the sun raises, by its heat, from the 

 atmosphere of the comet, when it approaches its 

 perihelion. He compares it to the smoke from 

 a burning body, which rises perpendicularly if 

 the body is at rest, or obliquely if the body is in 

 motion. In fact, the tails of comets, which al- 

 ways rise from the side which is opposed to the 

 sun, have a degree of curvature which is turned 

 from the side towards which the course of the 

 comet is directed. M. de Mairan attributes the 

 formation of the tails of comets to a part of the 

 solar atmosphere, with which he supposes the 

 comets to be charged, and which they draw 

 along with them in approaching their perihelion. 

 Other philosophers have supposed the tails of 

 comets to be collections of electric fluid, rendered 

 at once luminous and stationary. But all this is 

 mere conjecture. 



The number of the comets is certainly very 

 considerable. Riccioli enumerates 154, others 

 assert that 450 had been seen previous to the 



