316 METEOROLOGY. 



M 



tions were made. No similar table has before been derived from tlietn. 

 While it presents the analogies of other similar tables, it will be 

 better adapted to a large district of our country where meteorological 

 observations are being made systematically. Some gratification will 

 result to Professor Snell in such an application of his laborious and 

 scientific efforts, in this particular, in 1839. 



Such tables, it is evident, avail nothing where one or more times of - 

 observation, as sunrise or sunset, have a constant change, even though; 

 they may give an approximate mean. In different latitudes sunrise> 

 has different hours, as well as sunset, and the corrections must requirer 

 a far greater series of observations and far more labor. Though thei 

 four hours used at the military posts for several years give an approxi- '■ 

 mate mean, no correction for the sunrise observation is yet obtained.! 



So great is the labor of making the observations, and of discussing 

 them for practical purposes, that the fewest practicable hours, noti 

 exceeding three, should be adopted for the observations of meteor- 1 

 ologists generally. Only a few observers, who are favorably situated 'i 

 also, can afford to make hourly observations for a year or for years;; 

 and when such have been made, as enable observers to make the cor-- 

 rections from prepared tables^ the great object will be attained by 

 using only three hours of observation. The last line of corrections in i 

 the preceding table is derived from six hours of observation, used for' 

 some time at the Toronto Observatory, viz: six and eight a. m.j two 

 and/owr and ten p. m., and ituelve, or midnight. Though the correc- 

 tions are very small for these six hours, they are too numerous fori 

 ordinary object or advantage. The same objection lies against the 

 use of any four hours separated by six hours, as one and seven, both 

 a. m. and p. m.; which^ however, give very nearly the mean of 24 

 observations a day. Some of these hours will be very inconvenient 

 and troublesome. Take even the hours adopted by the Royal Society, 

 3 and 9 a. m., and 3 and 9 p. m. ; 3 a. m. is a very inconvenient hour, 

 though the four give very nearly the mean of the daily 24 observa- 

 tions, as shown in the first line of particular hours. 



In a series of observations of twelve years, like those in the "Army; 

 Meteorological Register" of 1856, these four hours, or any four hours,; 

 would require a million more observations than the three hours, be- 

 sides increasing the labor of the reductions one-third more than is ■ 

 necessary to attain the same approximation to accuracy. 



It is hoped that adequate evidence of the value of observations at 

 the hours 7, 2 and 9, has been presented, and that a near approxi- 

 mation to the true mean is attainable. The results may be corrected, 

 if need be, by the prepared tables. 



Rochester University, March 31, 1858. 



