40 Temperature of the Year. 
ponding straight line. ‘These intersections, which are seen to’ be 
a little after 9, A. M., and before 8, P. M., calculation fixes at 9h, 
5m. A. M., and 7h. 49m. P. M. — Hither of these points would be 
the proper time for a single daily observation, for the purpose of 
obtaining the mean temperature of the year. Ina paper in the 
Edinburgh Transactions, Vol. x., Dr. Brewster states the mean 
temperature of the years 1824 and 1825, to have occurred at 9h. 
13m. A. M. and 8h. 27m. P. M., in the latitude of Edinburgh. 
By the use of a maximum and minimum thermometer, I have 
registered the daily extremes of temperature, and the mean of 
those extremes. ‘The annual average of all the daily means is 
47°.38, which differs but slightly from the foregoing results. 
This method may undoubtedly be employed for obtaining. the 
annual mean with little error; but it is not accurate for all parts 
of the year; for in winter the true mean is lower, and in sum- 
mer higher, than the mean of daily extremes. 
The following rule for obtaining the mean temperature is 
adopted in the state of New York, by direction of the Regents 
of the University. Observe the thermometer first, between day- 
light and sunrise; second, between 2 and A, P. M.;. third, an 
hour after sunset. Add together the first, twice the second and 
third, and the first of the next day, and divide the sum by six. 
I have applied this rule to a few months of the year, and find 
the results to agree pretty nearly with those which are given 
above ; the greatest discrepancy noticed does not exceed three 
tenths of a degree. = 
That I might judge of the probable error arising from the omis- 
sion of the Sabbaths; I have compared together the mean tempera- 
ture at 9, A. M. and 3, P. M., taken from the meteorological jour- 
nal of the college, in which the record is made every day, and 
the mean temperature of the same hours, exclusive of the Sab- 
baths. ‘The former is 499.81; the latter 50°.01; making a dif- 
ference of one fifth of a degree. But the error arising from this 
source would affect to almost an equal amount both the annual 
mean, and the mean as deduced from the proposed system of two 
daily observations ; so that the rule applies with nearly the same 
precision as if the series had been uninterrupted, 
: 
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