METEOROLOGY. 187 
tropics itself, or 23° 28’, this maximum is about thirteen and a half hours, 
or three hours longer than the minimum. This difference is far more 
considerable at higher latitudes, so that the sun may, in a measure, make 
up by the length of time he remains above the horizon for what he loses by 
the obliquity of his rays. This duration of the longest day amounts to: 
14 hours at 30° 48’ 20 hours at 63° 23’ 
15 - 41° 24’ 21 sf 64° 50’ 
16 ‘6 49° 3! 29 66 65° 48/ 
17 e 54° 31’ 23 oy 66° 21’ 
18 ns 58° 27' 24 3 66° 32’. 
19 % 61° 19’ 
The longest day, with a slight allowance for refraction, is equal to the 
longest night of a place, and the shortest days and nights are found by 
subtracting the hours of the above table from twenty-four. The 
astronomical determination of the seasons is not quite appropriate for 
meteorology, this rather requiring spring for the northern hemisphere to 
begin March Ist; summer June Ist; autumn September Ist; and winter 
December Ist. The case is just the reverse in the southern hemisphere, 
mid-winter there corresponding to mid-summer here. 
In the torrid zone the year is divided into two seasons, the dry and the 
wet or rainy. The latter commences when the sun at noon approaches the 
zenith, and therefore at different times of year on the opposite sides of the 
equator. The previously clear sky becomes clouded, and rainy weather 
sets in, which continues with little interruption for several months. In the 
interior of Africa each of these seasons lasts just six months. In the frigid 
zone the year is likewise divided into two seasons, winter and sumnier, 
which yet are of very unequal duration, the former having the prepon- 
derance in proportion to the proximity of the pole. 
Nevertheless, the heat experienced at a given place, at a given time of 
the year or day, does not depend on the geographical position of the place 
or the situation of the sun alone, but also on many other circumstances. 
It is therefore of great interest to ascertain the variation of temperature of 
a place by continued and frequent thermometrical observations. If the 
thermometer, which for this purpose must be placed in the open air, and 
protected from the direct rays of the sun, and from radiation, reflection, 
&c., be examined hourly, it will be found that the minimum of daily 
temperature generally takes place fifteen or twenty minutes before sun-rise, 
the maximum some hours after noon, and later in summer than in winter ; 
in summer between two and three o'clock, in winter between twelve and 
one. The mean temperature of the twenty-four hourly observations is 
very nearly that of the highest and lowest temperature, or that of several 
corresponding hours in the morning and evening, as four and ten in the 
morning and evening. The temperature between nine and ten A. M., 
that at sunset, and that at eight P.M., is also very near the mean. 
Self-registering or maxima and minima thermometers are used to 
ascertain the highest and lowest temperatures in a given time, without the 
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