Boifal Astronomical Society. 69 



Dot succeed, ia arriving at aay satisfactory conclusion respecting the 

 cause and the laws of the irregularities. Aware of these anomalies, 

 which indeed were confirmed by his first observations, Mr. Henderson 

 a few weeks after his arrival at the Cape, undertook a laborious ex- 

 amination of the state of the instrument by means of a series of 

 readings of each of the six microscopes at every tenth degree of the 

 limb ; and in April and May 1833, shortly before his departure from 

 the Cape, he repeated the experiment on a more extended scale, by 

 examining the division corresponding to every fifth degree of the 

 circle, and also the divisions immediately before and after it. On 

 the results of this last experiment, which, however, were found to 

 be identical with those of the former one, he grounded the investi- 

 gation which forms the subject of the paper; — an investigation 

 which exhibits in a very advantageous point of view his sagacity, 

 patience and laborious accuracy, and is an admirable model of a 

 carefully conducted experimental inquiry. The result was, that in 

 order to explain the observed anomalies it was necessary-to suppose 

 the figure of the instrument to be an oval of small eccentricity, that 

 the pivots of the axis were not exactly circular, and that the whole 

 instrument frequently changed its position upon the pier from the 

 defective bearing of one of the pivots. But the most important 

 conclusion deduced from the investigation, — as it involved no less 

 important a question than the trustworthiness of the whole of his 

 Cape observations, — was that the mean of the readings of the six 

 equidistant microscopes was affected only to a very small extent (if 

 affected at all) by these imperfections, and that the probable error of 

 the instrument is not greater than the errors of the best instruments 

 of simTlar construction hitherto made. From a similar investigation, 

 founded on a less complete examination of the instrument by Mr. 

 Fallows, Mr. Slieepshanks and Mr. Airy, in a paper which is printed 

 in vol. v. of the Memoirs, had previously arrived at a similar con- 

 clusion*. 



While thus busily employed with the reduction of his own obser- 

 vations, on the results of which, he was well aware, his reputation 

 as an astronomer would essentially depend, Mr. Henderson's assist- 

 ance was still, as it had been in the early part of his career, freely 

 extended to others whenever an opportunity occurred of promoting 

 the cause of astronomy. Thus, at the request of Mr. Baily, he un- 

 dertook the reduction of Captain Foster's observations of the comet 

 of 1830, made at Ascension Island, the results of which, together 

 with an ephemeris for facilitating the calculation of observations of 

 the comet made in the southern hemisphere, are published in vol. viii. 

 of the Memoirs. Fortunately, however (even though thus turned to 



* It may not be without interest to state that, when this instrument was 

 sent back to England some years afterwards, and examined at the Royal 

 Observatory, it was found that, owing probably to some oversight in the 

 construction, the large steel collar carried by the conical axis was quite 

 loose. It is well, perhaps, the discovery was not made sooner, or the in- 

 strument would probably have been condemned, and the observations been 

 lost. 



