246 ASTRONOMY. 



its resistance. Should this material fluid revolve about the sun like a 

 vortex, it accelerates the revolution of each comet as have direct motions, 

 and retard those that have retrograde motions." 



ECLIPSES. 



"ALL uniformed persons regard eclipses with some degree of terror, 

 and among savages it is customary to make a great noise during the 

 occurrence of the phenomena, in order to prevent the evil which they 

 suppose would otherwise inevitably result. A late writer has very truly 

 remarked, that in times of ignorance, men are alarmed at all celestial 

 pnenomena, the recurrence of which takes place at periods too remote to 

 be readily calculated, and which accordingly appear to be guided by no 

 fixed laws ; whilst more important and obvious appearances excite no 

 surprise because they are frequent, and because they occur at well-known 

 periods : thus the changes of the moon are observed without alarm, while 

 the less obvious occurrence of an eclipse has scattered dismay over an 

 army or a nation. An incidental and considerable assistance has been 

 derived to chronology from this superstitious feeling. Eclipses were 

 thought to be connected, in some secret manner, with the destinies of 

 nations, and have been carefully recorded, when near the time of some 

 great battle or other political event, of whose epoch we should otherwise 

 remain in ignorance. Astronomical science shows how to determine 

 with the greatest exactness the hour of any given eclipse ; and we are 

 thus enabled to fix with precision the date of any event which may have 

 been thus accompanied, to confirm the statement of an historian, or to 

 correct his errors. But as the computation of eclipses is attended with 

 considerable difficulty, a few only of the readers of history are able to 

 carry on these researches for themselves. On this account catalogues 

 of eclipses have been calculated by astronomers for many thousand years, 

 by a reference to which any chronological point connected with these 

 phenomena may at once be determined. 



"Eclipses take place every half year, and at each period there may be 

 one, two, or three eclipses ; if one only, it must be an eclipse of the sun ; 

 if two, there will be one of each luminary; and if three, there will be two 

 of the sun, with one of the moon between them. 



" The sun is eclipsed whenever the moon comes between it and the 

 earth, so that the eye of a spectator on the earth is in a straight line with 

 the centres of the sun and moon. Sometimes what is called our annular 



