43 



in its origin, but has thus been carried far beyond utili- 

 tarian requirements. But the same mathematical skih 1 was 

 applied to other allied problems which were entirely in the 

 domain of pure science. The occasion for one of these was 

 the discovery, by PHILIBERT MELOTTE of the Greenwich staff, 

 of a very faint and distant satellite of Jupiter, the eighth 

 in order of discovery, and, as it proved, revolving round 

 its primary in the reverse direction to the seven previously 

 known. The calculation of the orbit of this satellite pre- 

 sented peculiar difficulties because, owing to the distance 

 of the satellite from its primary, the disturbing influence 

 of the sun was relatively very large. Another problem was 

 presented at about the same time by the expected return 

 of Halley's comet, and the need for determining in advance 

 the date of the approaching perihelion passage. These two 

 inquiries were undertaken by Cowell in close association 

 with ANDREW CROMMELIN, Superintendent of the Altazimuth 

 Department. By their combined labours the orbit of 

 Jupiter's eighth satellite was determined and Halley's comet 

 was followed backwards through its revolutions for more 

 than 2,000 years. Incidentally, a most important result 

 accrued from Cowell's part in these inquiries in that he was 

 able to devise new methods of dealing with such problems, much 

 shorter, more efficient and more easily controlled than those 

 which had previously been known. 



The 8th satellite of Jupiter had been first detected on a 

 photograph taken on February 28, 1908, with a reflector 

 of 30 niches aperture. This mirror is mounted in the largest 

 and most recent of the Greenwich buildings, and is carried 

 by the same mounting as three refractors. The largest of 

 these has an aperture of 26 inches and focal length of 22% feet, 

 and is corrected for the photographic rays. The smallest 

 is of 9 inches aperture ana 9 feet focal length, and is also 

 designed for photographic work. Both of these were pre- 

 sented by the late Sir Henry Thompson to the Royal Observa- 

 tory, and, with the equatorial stand and electrically-controlled 

 driving clock, are the work of HOWARD GRUBB of Dublin. 

 The third refractor, used as a guide for the 26-inch, possesses 

 an object-glass of I2| inches aperture and 18 feet focal length, 

 and was, from 1859 to I ^9 2 tne largest telescope in the Obser- 

 vatory. These two powerful instruments, the 3O-in. mirror 

 and the 26-in. photographic refractor, are used for photo- 

 graphing faint satellites of planets, faint minor planets, 

 comets, nebulae and double stars, and for the determination 

 of stellar parallaxes. The 3O-in. mirror is specially suitable 



