xvi President's Address 



their detection among the myriads of small stars is a matter 

 of considerable difficulty, even to accomplished observers ; 

 but, nevertheless, a systematic search for new members of 

 this group with telescopes of adequate power, appears to be 

 always rewarded by discovery. The "Lunar tables," as 

 they are called, are a series of numbers representing the 

 position, distance, &c., of the mooii from day to day or hour 

 to hour calculated for some years in advance, and are of the 

 utmost importance in practical astronomy, navigation, and 

 determination of geographical position generally. It is, 

 however, a remarkable fact that all tables hitherto computed 

 become erroneous after the lapse of years, so that the places 

 given no longer represent the moon's actual position, and this 

 would seem at first sight all the more remarkable because her 

 position is and has been continually observed by nearly all 

 the principal national observatories. But the complexity 

 of influences to which she is subjected in her motion through 

 space, coupled with the fact that her mass is probably phy- 

 sically unsymmetrical, makes it an extremely difficult pro- 

 blem to form a theory, taking all these disturbing influences 

 into account, so that tables founded on it shall give the 

 moon's precise position at very distant dates. The tables in 

 the American Nautical Almanac of Professor Pierce seem 

 however, to be the best yet computed. The veteran Astro- 

 nomer Royal of England, Sir George Airy, who is now in 

 his seventy-sixth year, has lately undertaken to work out a 

 new lunar theory to replace those which experience has 

 shown to be insufficient. He reports that his task is well 

 advanced towards completion, and I am sure all scientific 

 men at least will wish him health and vigour to complete 

 this great self-imposed task for the good of the whole civi- 

 lised world. 



In Physical Science also there is nothing of more than 

 ordinary interest to refer to. Mr. Crookes' investigations on 

 the action of light and heat on bodies in vacuo have been 



