DETERMININa THE LONGITUDE. 245 



This longitude agre^^s wrth that deduced from several Transits observed 

 about the same time, and is probably within 10% of the truth, 5 : 53: IJ. J. 

 being in excess. 



Such, briefly, is the process of deducing the Longitude from the transit of 

 the moon's limb, and stars near her. It obviously is the simplest, the most 

 convenient, and one of the most perfect methods that can be employed.* 



It is presumed, that every person who ventures to use the transit instru- 

 ment, has made himself acquainted with the elements of astronomy ; that he 

 knows the distinction between apparent and mean right ascension, and that 

 he is conversant with the uses of the tables, for giving the corrections for 

 precession, aberration, solar and lunar nutation. The tables used most com- 

 monly for this purpose, are those of Dr. Woollaston, but these are not so cor- 

 rect as could be desired. Correct tables will be found in DeLambre's Astro- 

 nomie Theorique et Pratique, also in Blot's Astronomic Physique, and in the 

 " Portable Tables," of Baron Zach, published at Florence in 1809. Neverthe- 

 less, all of these are tedious to ordinary computers, who do not always know 

 how to apply the negative and affirmative signs of an argument. The easiest 

 and most convenient are those prepared by the Reverend Mr. rallows,t con- 

 sisting of only five tables. One great advantage these tables possess is, that of 

 giving the correction for precession, without reference to the annual variations, 



♦ In the above example, the equation of Hid. differences is omitted, as being unimportant. 

 In the 38th Number of the Journal of Science and Arts, Mr. Henderson has inserted two con- 

 venient tables for finding the correction of Illd. and IVth. differences, 



•)- These tables are contained in Dr. Pearson's Introduction to Practical Astronomy, which con- 

 tains the most complete and useful selection of astronomical tables that has ever been published. 

 To underta!<e the compilation and publication' of so laborious, and so expensive an undertaking, re- 

 quired all the talents of that distinguished astronomer, and a degree of enthusiasm for science, of 

 wliicli there are but few examples. 



2 O 



