64 Royal Astronomical Societyim 



to recalculate the original data, and thereby detected the errors a»^ 

 not content with this, he submitted the entire process to a new cal-! 

 culation. His result differed immaterially from that which had been 

 previously obtained ; but the correction of the error, by rendering 

 the single results more consistent, gave a greatly increased confi- 

 dence to the general conclusion ; and, as was said of it by Sir John 

 Herschel himself, " had the effect of raising a result liable to much 

 doubt, from the discordance of the individual days' observations, to 

 the rank of a standard scientific datum, and thus conferring on a na- 

 tional operation all the importance it ought to possess." 



Mr. Henderson's connexion with this Society began by his under- 

 taking, upon the request of the Council, to compute an ephemeris 

 of the occultations of Aldebaran by the moon, in the year 1829, for 

 ten different observatories in Europe, In this undertaking he was 

 associated with Mr. Maclear, and the ephemeris, purporting to be 

 their joint production, was read at the December meeting in 1828, 

 and published in No. 15 of the Monthly Notices. His first contri- 

 bution to the Memoirs (published in vol. iv.) contained observations 

 of transits of the moon, and stars nearly in the same parallel of dcr > 

 clination, over the meridian, made at the Calton Hill Observatory in 

 1828, from which and corresponding observations made at Green- 

 wich he computed the difference of meridians. This paper deserves 

 notice, as showing that he had already adopted the practice of esti- 

 mating and allowing for the weights of the results and determining 

 their probable errors, according to the methods in use among the 

 German astronomers, but of which the examples were not, as yet, 

 frequent in this country. The method of determining differences of 

 longitude by means of observations of moon-culminating stars, then 

 recently proposed by Nicolai, had been strongly recommended by 

 Mr. Baily in a paper published in vol. ii. of the Memoirs, where all 

 the requisite rules and formulae were given for the computation. 

 Mr. Henderson entered into these views with his accustomed energy, 

 and not only embraced every opportunity of putting the method in 

 practice by computing corresponding observations, but was at much 

 pains to promote such observations by preparing lists of moon-cul- 

 minating stars for the use of observers. A list of this kind was pre- 

 pared by him at the request of the Council for the use of the 

 Arctic expedition under the command of Captain (Sir John) Ross 

 in 1830. 



Another subject on which his talents for computation were fre- 

 quently exercised in furtherance of the views of the Council, was the 

 calculation of the lunar occultations of fixed stars and planets. Ob- 

 servations of these phsenomena, interesting both to practical and 

 physical astronomy, were at that time much encouraged by the So- 

 ciety on account of their use in determining longitudes ; and, as such 

 phenomena will seldom be observed unless they have been predicted, 

 it was desirable to ascertain the times of their occurrence by a pre- 

 vious calculation. In this case also Mr. Henderson rendered most 

 efficient aid by contributing, for several years, monthly lists of the 

 principal lunar occultations computed for the meridian of Greenwich. 



