ERA OP NEWTON, HALLEY, AND HERSCHEt. 35 



Roemer, the inventor of the transit instrument with which he made observations from 

 the window of his house, rendered no unimportant service by showing that the instru- 

 ments need not be fixed on high towers: he also discovered, in the year 1675, the interesting 

 and hitherto unsuspected fact, of the progressive transmission of light through Space, and 

 the appreciable velocity with which it travels. This was attained by a series of careful 

 observations of the eclipses of Jupiter's satellites. It was found, by comparing the times 

 of immersion of the satellites in the planet's shadow and emersion from it, with the times 

 calculated from the laws of their movements, that there was 

 an acceleration or retardation of the phenomena by a few 

 minutes, plainly dependent upon the variations of the earth's 

 distance from Jupiter ; for the retardation was observed to be 

 the greatest when the earth was in that part of its orbit most 

 remote from him. The diameter of the orbit of the earth 

 being a hundred and ninety millions of miles, we are more 

 remote from Jupiter, by the whole of that distance, at one 

 time than at another ; as, when the earth is in its orbit at , 

 its distance is greater from j than when at b by the interval 

 between the two points. But notwithstanding this immense addition of space, or any 

 conceivable increase, an eclipse would be observed to occur no later at the one than 

 at the other, if light were propagated instantaneously. Roemer found, however, a differ- 

 ence of eleven minutes to exist, which he afterwards estimated at fourteen, but which the 

 precision of modern astronomy has fixed at sixteen minutes and a quarter. This deter- 

 mines the progressive motion of light, and the rate of its velocity. It requires time for 

 its transmission ; and flying over the diameter of the earth's orbit in sixteen and a quarter 

 minutes gives it a velocity of twelve millions of miles a minute, or upwards of a hundred 

 and ninety thousand miles a second. Thus, in the eighth part of a second, it accom- 

 plishes the passage of a space equal to the equatorial circumference of our globe : yet so 

 vast is the system to which we belong, that this swift-winged messenger, which requires 

 no more than two hours to travel from the central sun to the farthest planet, could 

 not dart through the intervening solitudes between us and the nearest of the stars 

 under a period of five years. Notwithstanding the velocity of the rays of light, which 

 travel more than fifteen hundred thousand times faster than a cannon ball, experi- 

 ment has not yet been able to detect that they have any impulsive power. The 

 surmise has, however, been thrown out and it is not improbable that the attri- 

 tion of the solar beams with the terrestrial surface may have some connexion with the 

 phenomena of heat. 



The national observatory of England the noblest institution in the world for the 

 extent and exactitude of its astronomical tables, and their practical value in the art of 

 navigation was originated by the spread of foreign commerce. The growth of our colo- 

 nies across the Atlantic, together with the establishment of our relations with India, ren- 

 dered it of the first importance to have an easy and accurate method of finding the longi- 

 tude at sea. A plan was proposed, founded upon the principle now in use, of observing 

 the lunar motions and distances during a voyage, and comparing them with a previous 

 home calculation, thus ascertaining the difference between home time and time at sea, 

 from whence the difference of longitude is readily deduced. A reward being sought by 

 the proposer from the government of Charles II., it was referred to a commission to 

 report upon the merits of the scheme. Flamstead, one of the commissioners, at once 

 decided against its practical utility, on the ground of the inaccuracy both of the lunar 

 tables and of the positions of the stars in existing catalogues, which only a lengthened 

 course of observation could rectify. The king, declaring that his pilots and sailors 



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