102 PLEASANT WA YS IN SCIENCE. 



of the waves is similarly doubled at this most favourable 

 time for observation ; and the actual difference between the 

 motion of the two sides of Jupiter's equator being nearly 

 15 miles per second, the effect on the light-waves is 

 equivalent to that due to a difference of nearly 30 miles 

 per second. Thus the new method may fairly be expected 

 to indicate Jupiter's motion of rotation. The Greenwich 

 observers have succeeded in applying it, though Jupiter has 

 not been favourably situated for observation. Only on one 

 occasion, says Sir G. Airy, was the spectrum of Jupiter 

 " seen fairly well," and on that occasion " measures were 

 obtained which gave a result in remarkable agreement with 

 the calculated value." It may well be hoped that when in 

 the course of a few years Jupiter returns to that part of his 

 course where he rises high above the horizon, shining more 

 brightly and through a less perturbed air, the new method 

 will be still more successfully applied. We may even hope 

 to see it extended to Saturn, not merely to confirm the 

 measures already made of Saturn's rotation, but to resolve 

 the doubts which exist as to the rotation of Saturn's ring- 

 system. 



Lastly, there remains the rotation of the sun, a move- 

 ment much more difficult to detect by the new method, 

 because the actual rate of motion even at the sun's equator 

 amounts only to about i mile per second. 



In dealing with this very difficult task, the hardest which 

 spectroscopists have yet attempted, the Greenwich observers 

 have achieved an undoubted success ; but unfortunately for 

 them, though fortunately for science, another observatory, 

 far smaller and of much less celebrity, has at the critical 

 moment achieved success still more complete. 



The astronomers at our National Observatory have been 

 able to recognize by the new method the turning motion of 

 the sun upon his axis. And here we have not, as in the 

 case of Venus, to record merely that the observers have 

 seen what they expected to see because of the known 

 motion of the sun. " Particular care was taken," says 



