36i 



Longitude d^terttltned hy the Moon's Tranfits. 



Although we can fafely truft to the accuracy of the dh^ 

 fervations at the roval obfervatory, yet, at thedidant ftation /3, 

 ffreat attention inuft be paid in adjufting the tranfit inftni- 

 ment truly to the meridian, and taking care that it be perteft 

 iii collimation before the moon's tranfit be obferved there ; 

 arid alfo, that correa time be deduced as foon before or after 

 the moon's tranfit as poflible, from obfervmg the meridian 

 naffacre of one or more of the thirty-fix ftars, whofe right al- 

 cenfioa>, after a long feries of obfervations, have been alcer- 

 twined by the Aaronomer Royal with the greateft precifion. 



In order tA put the above method to the teft 6f experiment, 

 a number of tranfits of the moon's firft limb were obferved 

 jnanv years ago at a place 26 feconds of time weft from the 

 royal obfervatory, with a fmall portable tranfit inftrument 

 made by that eminent artift Mr. Edward Troughton, of Fleet- 

 ftreet. It had a telefcope ao inches long, a treble objeA- 

 glafs, and the price only 20 guineas. When the obfervations 

 at Greenwich were publiflied, all thofe that were made on 

 the fame days at both places were employed to determine the 

 difference of longitude, and the mean refult of four or five 

 correfponding obfervations gave it true to two feconds of time. 

 This is only mentioned to (liow, that with fmall tranfit in- 

 ftruments well conftrudled, and due attention paid to their 



adjuft- 



