60 The Velocity of Light. 



between the blows before it. If the person were, after having gone 

 a certain distance, to turn round, the interval between the blows 

 would become gradually less and less. If instead of walking 

 straight away and straight back, he were to walk in a circle, start- 

 ing from a point just opposite the man who was using the hammer, 

 the interval between the blows would appear to get longer and 

 longer till he reached the point in the circle exactly opposite to 

 that from which he started, and after that they would get shorter 

 and shorter till he reached his point of starting. The circumstances 

 with regard to the satellite of Jupiter are precisely similar. At that 

 part of the earth's orbit where the earth is opposite and nearest to 

 Jupiter, the interval between each eclipse is 42 47 hours, and the 

 interval between the eclipses gradually increase, till, at the time when 

 the earth is further off from Jupiter, than at its point of starting by 

 the distance of the diameter of the earth's orbit, the increase of 

 interval between the eclipse is sixteen minutes, thirty six seconds, 

 (according to Roemer's imperfect observations 22 minutes,) which 

 proves that light takes that time to cross the diameter of the 

 earth's orbit, and if we apply our rule, and divide the distance 

 which is about 190,000,000 miles by the time, we will find the 

 result to be that light travels about 190,000 miles per second; this 

 calculation, though not very far from the truth, is now proved 

 not to be perfectly correct. This explanation of the unequal 

 intervals between the immersion of the first satellite of Jupiter 

 was not universally accepted at the time, many people main- 

 taining that they were due to a real disturbance of the satellite 

 itself, due to some cause unknown, and that there was some 

 confirmation needed, a confirmation which was soon furnished 

 by the discovery of the aberration of the stars by Bradley. The 

 phenomenon of aberration consists in the apparent displacement 

 of stars and planets, caused by the combination of the velocity 

 with which light moves, and the velocity with which the earth 

 moves. We can draw an illustration of this from the ordinary 

 occurrences of nature. Suppose for instance we are in a square 

 waggon open at both ends, during a shower of rain in which there 

 is no wind, and in which therefore the drops fall down per- 

 pendicularly ; if the waggon is stationar}', the drops will appear, 

 as is really the case, to be falling down perpendicularly, but if we 

 move in any direction the drops will appear to be coming down in 

 a slanting direction, the slant being greater and greater as the 

 waggon goes faster and faster. Now let us follow the course of a 

 single drop, let us first suppose that the waggon and the drop are 

 both travelling at the same pace, then it is perfectly evident, since 

 the waggon is square, that if a drop just entered in at the top of 

 the waggon it would just go out at the bottom at the opposite end ; 



