J 



S6 Boivditch on correcting the ajpparent distance of the Moon k 



Th 



th 



precetli 



pi 



we have E 



F 



S', 



hence the fourth correction in Tab 



G 



i^. The fifth correc- 



tion iu Table H is less than a second. Both these 

 insensible ; and this is generally the case except t 

 the objects is very small, and the altitudes very low 



ly made in such 



'; 



distance of 

 but observa- 

 at of the un- 



certainty of the refraction near the horizon^ whirh is a much great- 

 er source of error than the corrections now under consideration. 

 For a variation of ten degrees in Fahrenheit's thermometer would 



produce an alteration of 



// 



refraciion of a body, situated 



above the horizon 5 this might produce a correction of the same or- 

 der in the true distance, which would in general be much greater 

 than the sum of the fourth and fifth corrections. Now as Naviga- 

 tors do not usually notice the corrections depending upon the ther- 

 mometer and barometer, it becomes necessary to avoid those ob- 



corrcctions for temperature and density 



which the 



would be great, or in other words, the low altitudes. But if the 

 objects are sufBcieutly elevated to render the corrections for the 

 temperature and density small, the fourth and fifth corrections 

 will be hardly sensible, so that for all practical purposes, it will 

 be suflicient to notice the rest of the corrections in the preceding 

 formula (A) and neglect the two last. In addition to this we may 

 observe that the quantities thus neglected are not in general great- 



^ 



er tliau those depending upon the spheroidal form of the earthy 

 "Which are rarely, if ever^ taken into consideration ; neither are 

 they greater than the errors to which the lunar tables are liable^ 

 and it appears to be an unnecessary degree of accuracy to notice 



equations which are within the limits of the errors of those tables. 



«i» 



/ 



/ 



K 



