HISTOBY OF PHYSICAL ASTRONOMY.
405
had never been observed at all. The altitude and azimuth instrument,devised by Mr. Airy for remedying this defect, was constructed by thejoint exertions of Messrs. Eansome and May, engineers of Ipswich, andMr. Simms of Loudon, the successor of the celebrated Troughton. Boththe vertical and azimuthal circles are 3 feet in diameter. The telescopeattached to the instrument is 5 feet long, and is furnished with an object-glass of 3f inches aperture * * * § . The observations with it were commencedon the 10th of May, 1847. In practice it has been found to work admi-rably. Mr. Airy considers the results to be hardly, if at all, inferior tothose obtained by the use of the mural circle. The utility of the instru-ment in promoting the object for which it was designed, soon became ap-parent. It has been found that the moon may be observed with it in themorning and the evening, when she is only an hour distant from the sun.The consequence has been, that the number of lunar observations, con-tained in the volume that issues annually from the Greenwich Observa-tory is now about three times greater than it formerly was I, while, at thesame time, the observations with the new instrument, possess this peculiaradvantage, that many of them have been made in a part of the moon’sorbit, where no observations at all had hitherto been supposed to be prac-ticable J. It is manifest that this circumstance cannot fail to be attendedwith advantage in contributing towards the further improvement of thelunar theory.
The complete reduction of the observations of the moon and planetsmade at Greenwich since the middle of the last century, is a work withwhich the name of the present Astronomer Royal is imperishably associated.At the meeting of the British Association, held at Cambridge in 1833, Mr.Airy suggested the expediency of effecting a uniform reduction of all theobservations of the planets made at Greenwich since 1750, the year whenBradley commenced his observations with the new instruments con-structed by Bird §. He proposed, on that occasion, that if the Governmentshould agree to defray the necessary expense, he would undertake gratui-tously to superintend the whole operation. At the instance of the Associa-tion, the Government acceded to this proposal, and the work was forthwithcommenced. The results were finally published in 1846, in one largequarto volume. The whole work is divided into five sections. In thefirst section the errors of the clock are deduced from the observations.In the second the errors of the instrument are similarly investigated. Inthe third the geocentric places are computed from the observations. In
* A detailed description of this instrument will be found in the volume of the Green-wich Observations for 1847.
f During the first year which elapsed after the instrument was in operation, the numberof lunar observations made with it amounted to 203. The number of similar observa-tions made on the meridian during the same period was only 111.
J This must be understood only with respect to the sun. The moon had beenobserved from the earliest ages in every part of her absolute orbit; but as the perturba-tions of her motion depend mainly on her angular distance from the sun, it is obviouslya matter of primary importance that she should be observed in every part of her courserelative to that body.
§ Previous to the epoch referred to in the text, astronomers were not in the habit ofrecording their observations so fully as to admit of their being reduced in the present daywith all desirable accuracy. Thus the indications of the barometer and thermometer hadnot been hitherto noted by any observer, not even excepting Bradley. Apart, therefore,from all consideration of the superiority of the instruments constructed by Bird, this cir-cumstance alone would render it inexpedient to attempt the reduction of any anterior ob-servations.