PHYSICAL SCIENCE. 



XXIX 



tha Principia, and states the universal shout of 

 admiration, which, after a long interval of years, 

 was raised from every country in its praise. But 

 nothing conformable to the Newtonian principle 

 had at that period appeared in France ; we sus- 

 pect, indeed, that we might say that nothing con- 

 formable to it had appeared in Europe. 



It is curious, and contributes not a little to 

 lead -us into the character of our lively neigh- 

 bours, that the great apostle of Newton in France, 

 the person who contributed most to make his 

 opinions known and adopted, was Voltaire. 



Fontenelle continued a Cartesian to the end 

 of his days. Cassini and Miraldi seem to have 

 been quite unacquainted with the Newtonian 

 system, and continued the most vague and im- 

 aginary hypotheses for calculating the paths of 

 comets, after Halley had computed tables from 

 which the motions of all the comets that had ever 

 appeared, or ever could appear, might be easily 

 deduced. 



We must now proceed to notice the most im- 

 portant additions which have been made to 

 astronomy since the appearance of the Principia. 



1. The aberration of the Fixed Stars. Drs 

 Bradley and Molineux, in the end of the year 

 ] 725, were occupied in searching for the parallax 

 of the fixed stars by means of a zenith sector, 

 which was erected at Kew. It was of great 

 diameter, and furnished with a telescope twenty- 

 four feet long, with which they proposed to ob- 

 serve the transits of stars near the zenith, accord- 

 ing to a method first suggested by Hooke, and 

 pursued by him so far as to induce him to think 

 that he had actually discovered the parallax of 

 y Draconis, the bright star in the head of- the 

 dragon, on which he made his observations. 

 They began to observe the transits of the same 

 star on the 3d of December, when the distance 

 from the zenith at which it passed was carefully 

 marked. By the observations of the following 

 days, the star seemed to be moving south ; and 

 about the beginning of March in the following 

 year, it had got 20" to the south, and was then 

 nearly stationary. In the beginning of June it 

 had come back to the same situation where it 

 was first observed; and from thence it con- 

 tinued its motion northwards till September, 

 when it was about 20" north from where it was 

 first seen ; its whole declination amounting to 

 40". 



This motion surprised the observers a good 

 deal ; as it lay the contrary way to what it would 

 have done had it proceeded from the parallax of 

 the star. But the repetition of the observations 

 proved their accuracy. They were afterwards 

 continued by Bradley, with another sector of 

 smaller radius, but large enough for the purpose 

 which embraced a larger arch, and admitted the 



observation of stars which passed at a greater 

 distance from the zenith. Even with this addi- 

 tion, the observations did not put Bradley in pos- 

 session of the complete fact. They only gave 

 him the motion of each star in declination, but 

 not in right ascension. 



After considering the subject with the greatest 

 attention, and excluding all those causes which 

 were inadequate to produce the effect, it occurred 

 to him that the apparent motions might be pro- 

 duced from the progressive motion of light, com* 

 bined with the motion of the earth in its orbit. 

 If the earth were at rest, it is evident that a 

 telescope, to admit a ray of light from a star to 

 pass along its axis, must be directed to the star 

 itself. But if the earth (and of course the teles- 

 cope) be in motion, it must be inclined forward, 

 so as to be in the diagonal of a parallelogram, 

 the sides of which represent the motion of the 

 earth and the motion of light, or in the direction 

 of these motions, and in the ratio of their velo- 

 cities. It is with the telescope just as with the 

 vane at the mast-head of a siiip. When the ship 

 is at anchor the vane takes exactly the direction 

 of the wind ; but when the ship is under way it 

 places itself in the diagonal of a parallelogram, 

 of which one side represents the velocity of the 

 ship, and the other the velocity of the wind. 



The telescope, therefore, through which a star 

 is viewed, and by the axis of which the position 

 of the star is determined, must make an angle 

 with the straight line drawn to the star; except 

 when the earth moves directly to the star, or 

 directly from it Hence it follows, that if the 

 star be in the pole of the ecliptic, the telescope 

 must be pointed forwards in the direction of the 

 earth's motion, always by the same angle ; so 

 that the star would be seen out of its true place 

 by that angle, and would appear to describe a 

 circle round the pole of the ecliptic, the radius of 

 which subtended at the earth an angle, of which 

 the sine is to unity as the velocity of the earth to 

 the velocity of light 



These velocities Bradley took at 1 to 10313, 

 as most suitable to his observations, which made 

 the radius of the circle of aberration 20", and the 

 transverse axis of the ellipse, or the whole change 

 of place, 40". It was the shorter axis which 

 Bradley had actually observed in the case of 

 y Draconis, that star being very near the solstitial 

 colure, so that its changes of declination and of 

 latitude are almost the same. 



To show the truth of his theory, ho computed 

 the aberration of different stars, and, on com- 

 paring the results with observation, the coinci- 

 dence appeared almost perfect, so that no doubt 

 remained concerning the truth of the principle 

 on which he had founded his calculations. He 

 did not explain the rules themselves Clairaul 



