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LX. Onjinding the exact Mean Solar Time. By M. Smith, 

 Esq. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 TN reply to a letter published in your last Number (p. 210), 

 ■*■ I beg leave to inform your correspondent Mr. Cooper, that 

 in computing the hour of the day from the proposed data, viz. 

 the latitude of the place, the sun's declination and altitude ; he 

 must invariably use the visible latitude, or that deduced im- 

 mediately from observation ; the reduced latitude being no- 

 thing more than a fiction invented for the purpose of facilita- 

 ting the computation of solar eclipses, or other occultations of 

 the heavenly bodies by the moon. 



I have carefully examined the calculation your correspond- 

 ent has made, and find it perfectly correct, except the line 

 marked with an asterisk, which is unnecessary; the effect of 

 this proportion would be to allow twice for the variation of 

 the equation of time between noon and the instant of obser- 

 vation, which variation ought to be taken only once into the 

 account, as is done by correcting this equation to the instant 

 of observation. The latter part of the computation ought, 

 therefore, to stand thus : 



Apparent solar time 3 h 26 m 14 s, 9 



Equation of time corrected to that instant 1 47 '9 



Mean solar time 3 28 2 «8 



The right ascension of the sun and stars as given in the 

 Nautical Almanack, is expressed in sidereal time. 



In using Dr. Tiarks's tables, the equation of time need not 

 be considered at all ; the reason of which is, that the interval 

 between the sun and the star passing the meridian is given in 

 mean solar time, which is not subject to any inequality. It 

 must be observed, however, that to apply these tables to the 

 year 1825, table I. must be corrected, by adding 2' 59 7 in the 

 months of January and February, and subtracting 57" during 

 the remainder of the year; the right ascensions of the stars in 

 table III. require also to be augmented by their annual varia- 

 tion. I remain, gentlemen, 



Your most obedient servant, 

 October 13, 1824. M. SMITH. 



LXI. On 



