244 DETERMINATION OF THE LONGITUDE OF SEVERAL. 



London, by Francis Bailey, in a paper containing an original method 

 for their reduction. This zealous astronomer selected lists of moon- 

 cuiminating stars, and caused them to be distributed in advance to 

 every observatory. The labours of Bessel, Hansen, Molhveide, Du- 

 mouchel, and others, have further developed the method of making 

 and reducing this kind of observations. But the principal improve- 

 ment was made in this method by the Nautical Almanac Committee, 

 in causing the announcements of moon-culminations to be made in such 

 a form, since 1834, that the reduction of them consists merely in the 

 interpolation of the series there given for the right ascension of the 

 moon's bright limb, at its upper and lower culminations at Greenwich. 

 These phenomena are now regularly observed at the British and 

 continental observatories, and their longitude from each other has been 

 determined by this method with an accuracy scarcely inferior to that 

 of geodetic measures, powder signals, or the aggregate of observed oc- 

 cultations and eclipses. The observations for longitude, made by Cap- 

 tain Talcottj consist of one occultation, and three series of moon-cul- 

 minations, at three different stations. In making the latter, a portable 

 transit instrument was used, of three and a half feet focal length, and 

 two inches and five-eighths aperture. Care was taken, previous to 

 each moon-culmination, to adjust the horizontal axis by a delicate level, 

 and the bias of the instrument was therefore as small as a temporary 

 mounting would permit. The line of collimation of the instrument 

 was adjusted for the mean of the wires, and does not appear to have 

 undergone sensible change during the series, though frequent observa- 

 tions were made with the reversed axis, to detect the error, if any, in 

 this adjustment. The deviation in azimuth was ascertained by ob- 

 serving high, low, and circumpolar stars, and a temporary meridian 

 mark served to give steadiness to this adjustment. The sura of the 

 deviations was usually less than 0-5 sec. in time. The results have 

 been corrected for this sum, as far as it could be ascertained. It is 

 difficult, with an instrument temporarily mounted, to furnish a greater 

 degree of precision. The error arising from deviations so small is al- 

 most insensible, in the longitudes deduced from moon-culminations. 

 The times were noted by calling out to assistants, and were registered 

 on two chronometers by Brockbank, the assistant noting to the nearest 



