ECLIPSE OF JUPITER'S SATELLITE. 31 



nomenon most convenient for such purposes as this is an 

 eclipse of some one of the satellites of Jupiter. 



VELOCITY OF LIGHT ASTRONOMICAL DETERMINATION. 



In the engraving, e represents the satellite about enter- 

 ing the shadow of the planet. The precise moment at 

 which it really enters is known beforehand, and it is found 

 by accurate observation that the apparent time of its en- 

 tering, as seen from the earth when it is at T, in that part 

 of its orbit which is nearest the planet, is a certain number 

 of minutes sooner than when it is observed when the earth 

 is at t that is, in the part of its orbit which is most re- 

 mote. From these data, the time required for the light to 

 pass through the diameter of the earth's orbit is deter- 

 mined. 



The conclusions deduced from astronomical observations 

 like these have been abundantly confirmed by ingenious 

 devices which have been contrived for measuring the ve- 

 locity of light on the earth's surface. The engraving on 

 the following page represents one of these methods, the 

 principle of which, with a little attention, can be easily un- 

 derstood, though it would require a great deal of practical 

 experience and skill, and very delicate powers of observa- 

 tion, to enable any person to perform the experiment suc- 

 cessfully with it, so as to arrive at a satisfactory result. 



You will see by the engraving that the apparatus con- 

 sists of two separate parts, which are connected by dotted 



