^ 



Rev. Mr. Emmett on 



[Jan. 



because it gives indications of a gale considerably before its 

 arrival), the correction may be easily applied, and will add to 

 the accuracy of the observation. "But in general the common 

 tables of refraction and parallax will suffice. 



Having applied the corrections, and found the angle Z, which 

 angle is not altered by the corrections, the next step is to find 

 the arc s m ; and the difference, viz. s w — S M, or S M — s m, 

 is the quantity to be registered in the tables, and which is to be 

 added to or subtracted from S M, the measured distance, to 

 reduce it to the true. In the spherical triangle, Z i m, Z s, Z m, 

 and / Z, are given ; s m is required. To the nat. v sin. of the 

 difference of the sides, add the number 

 corresponding to the log. v sin. of the angle 

 z + log. sin. Z m + log. sin. Z s; the sum 

 is the nat. v sin. s 7n. These are the data on 

 which I mean to construct the tables. 



Let Z M be a zenith distance of the 

 moon, the distance M S being about one- 

 fourth part less than the diameter of the 

 field of view of the telescope ; with radius 

 M S, and from M, describe a small circle 

 S S' S". Suppose MS = 2° ; Z S'^' = 

 Z M - 2°, and Z S'^ = Z M + 2° ; and 

 the other values of Z S, Z S', Z S", are 

 intermediate; to wit, Z S'^' = Z M - M S; 

 ZS = ZM- ZS + 10'; ZS'=ZM-ZS 

 + 20', &c. For all these cases, the cor- 

 rection will be registered, which, added to or subtracted from 

 M S, will reduce it to the true distance, or clear it from the 

 effects of refraction and parallax. Diminish M S by 15', Z M 

 and Z S having the same values as before ; and calculate the 

 corrections until by continually subtracting 15', M S becomes 

 equal to the moon's seraidiameter. The following is a specimen 

 of two pages of the tables. 



