OF ARTS AND SCIENCES. 381 



in the position of the stars. Suddenly the explanation was flashed 

 upon him by something he observed while yachting on the River 

 Thames. He noticed that, whenever the boat turned about, the 

 direction of the vane altered. He asked the sailors, Why ? All they 

 could say was, that it always did. Reflecting upon the matter, Brad- 

 ley concluded that the motion of the boat was compounded with the 

 velocity of the wind, and that the vane represented the resultant direc- 

 tion. He was not slow in seeing the application of this homely illus- 

 tration of the parallelogram of motion to his astronomical puzzle. The 

 velocity of light was compounded with the velocity of the earth in its 

 orbit, so that its apparent direction differed by a small angle from its 

 true direction, and the difference was called aberration. In spearing 

 a fish or shooting a bird, the sportsman does not aim at them, but 

 ahead of them. This inclination from the true direction is similar, in 

 angular measure, to what the astronomer calls aberration. Struve's 

 measurement of aberration combined with the velocity of the earth in 

 its orbit gave for the velocity of light 191,513 miles a second. Both 

 of the two methods described for obtaining the velocity of light depend 

 for their accuracy upon the assumed distance of the earth from the 

 sun. The distance adopted was the one found by the transits of Venus 

 in 1761 and 1769, viz. 95,360,000 miles. 



During the last forty years, the opinion has been gaining ground 

 amon<r astronomers that the distance of the sun, as deduced from 

 the transits of Venus in 1761 and 1769, was too large by three per 

 cent. Expeditions have been sent to remote parts of the earth for 

 observing the planet Mars in opposition. The ablest mathematical 

 astronomers, as Laplace, Pontecoulant, Leverrier, Hansen, Lubhock, 

 Airy, and Delaunay, have applied profound mathematical analysis to 

 the numerous perturbations in the planetary motions, and proved that 

 the sun's distance must be diminished about 2,000,000 miles in order 

 to reconcile observations with the law of gravitation. Airy reduced 

 the distance of the sun by more than 2,000,000 miles, to satisfy the 

 observations on the transit of Venus in 1874. Glasenapp derived 

 from observed eclipses of Jupiter's satellites a distance for the sun of 

 only 92,500,000 miles. From these and similar data, Delaunay con- 

 cluded that the velocity of light is about 186,420 miles a second. 



These triumphs of astronomical theory recall the witty remark of 

 Fontenelle, that Newton, without getting out of his armchair, calcu- 

 lated the figure of the earth more accurately than others had done by 

 travelling: and measuring to the ends of it. And Laplace, in contem- 

 plation of similar mathematical achievements, says : " It is wonderful 



