1830-40] ROYAL ASTRONOMICAL SOCIETY 75 double stars, Dawes, began his labours on these objects about 1830.* The brilliant discovery by W. Herschel of binary stars naturally led to attempts being made to calculate their orbits as soon as a sufficiently long arc had been described. The first to do so was Savary in the Connaissance des Temps for 1830, and he was soon followed by Encke in the Berliner Jahrbuch for 1832. Both their methods were perfect from an analytical point of view ; but J. Herschel considered it a great objection to both, that they required four complete measures. At that time it was assumed that position angles could be measured without much danger of systematic errors, while this was not supposed to be the case with the distances. Herschel therefore rejected the use of distances (except for the determination of the major axis), and found the elements by a happy combination of graphic construction and numerical calculation. f HerscheFs examination of the northern heavens was completed in 1833 May, and as soon as his preparations could be finished (even before all his previous observations were ready for publica- tion) he embarked with his instruments for the Cape of Good Hope, in order to extend to the southern hemisphere the review which his father and he had made of the northern sky. Landing at Cape- town in 1834 January, he lost no time in erecting his instruments in a suitable locality about six miles from the town, so that he could begin regular work on March 5. The last " sweep " with the 20 -foot reflector was made on 1838 January 22, and thus was brought to a close an undertaking which is unique in the history of science, having been carried out in the course of thirteen years by one individual without any help whatsoever, and entirely at his own cost, including an expedition to a distant part of the earth lasting four years. No wonder that he was honoured in many ways on his return to England in the spring of 1838 ; his scientific friends and admirers gave him a hearty welcome at a festive banquet, before he settled down to the laborious task of preparing for publication the immense number of results of his expedition. Before Herschel left Slough in the spring of 1840 to spend the remainder of his life in Kent, he had to dismount his father's famous 40 -foot telescope, the woodwork of which had become dangerously decay ed.J This was done in 1839 December, a date which is of importance, as it serves to fix the time of a great advance
- Memoirs, 5, 135. *39 ; 8, 58, 61.
t Ibid., 5, 171 ; further applications of the method, 6, 149. Herschel returned to the subject many years afterwards in volume 18. J The " Requiem," written by J. Herschel and sung inside the tube on New Year's eve, 1839-40, is printed in Weld's History of the Royal Society, 2, 195, and in the Astronomische Nachrichten, No. 405 (Bd. xvii.).
