The Velocity of Light. 69 



phenomena exhibited in the periodical eclipses of the first satellite 

 of Jupiter. In order that we raay fully understand Roemer's dis- 

 covery I must give a short account of Jupiter and its satellites or 

 moons. Jupiter is the largest of the planets, and revolves like our 

 earth, round the sun. It possesses four satellites which revolve 

 round it much in the same manner as it revolves round the sun. 

 Now since none of the planets or their satellites have a proper light 

 of their own, but only a reflected light from the sun, it is quite 

 evident that if any obstacle interposes itself between them and the 

 sun's light they will be invisible, or in other words there will be an 

 eclipse of that particular planet or satellite. Such being the case, 

 whenever one of Jupiter's satellites gets into the cone of the shadow 

 of Jupiter that satellite will be eclipsed. And this happens to all 

 of Jupiter's satellites, except the fourth, which occasionally passes 

 above or below the shadow, in every revolution. 



These eclipses happen much more frequently to the first of 

 Jupiter's satellites than to any other, owing to its being much 

 nearer Jupiter, and therefore describing its orbit in less time. 

 The movements of this satellite round Jupiter are very regular 

 and therefore the eclipses occur almost without any deviation once 

 in every 42.47 hours. Owing to our always seeing the cone of 

 Jupiter's shadow obliquely, the exact moment of both the immer- 

 sion or entering in of the satellite, and the emersion or coming 

 out of satellite can never both be seen in the same eclipse. Either 

 we see the exact moment of the immersion, and not that of the 

 emersion, or else we see the exact moment of the emersion 

 and not that of the immersion.* To illustrate Roemer's great 

 discovery, let us again refer to the phenomena exhibited in the 

 transmission of sound through our atmosphere ; suppose that we 

 again have our man who is striking a piece of iron as our source 

 of sound, and let us suppose that there is an equal interval of time 

 between each blow. Then let us suppose a person standing so 

 close to the source of sound, that the sound of the hammer hitting 

 the iron appears to coincide with the descent of the hammer upon 

 the iron ; if the person were then gradually to walk away, the in- 

 terval between the sound of each blow reaching him would be a 

 little longer, because between the delivery of each blow he has 

 gone a short distance, and therefore, the sound must take a little 

 longer to reach him, the little longer being the time sound takes 

 to travel the distance he has walked, between the delivery of the 

 two blows. If the person did not know that the blows were de- 

 livered with perfect regularity, he would naturally suppose that 

 there was a slightly longer interval between each blow, than that 



• Most of these facts were discovered by Galileo's telescope and were ingeDiously 

 applied by Roemer to the problem before us. 



