194 PHYSICS. 
general, however, from observations made on both hemispheres, from 0° to 
79° of latitude, and to a height of 12,000 feet, it has been found that the 
barometer twice reaches a maximum and twice a minimum every day. The 
maxima occur between 8} and 103 a. mM. (on an average at 9h. 37m.), and 
between 9 and 11 p.m. (average 10h. 11m.); the minima between 3 and 5 
A. M. (average 37 a. ™.), and between 3 and 5 p.m. (average 4h. 5m.). 
These hours of highest and lowest barometer stand in the course of a day, 
where the rise of the mercury exceeds the fall, or the reverse, are called the 
hours of turn; they fall nearer noon in winter than in summer. In winter, 
and during the rainy season of the tropics, the daily variations are least, and 
greatest in April. On high mountains in the torrid zone, the diurnal 
variations are much less than in the low lands. Here there is not, asa 
general rule, a twofold rise and fall of the barometer, but only one daily 
maximum and minimum; the hours of turning also lie nearer noon. The 
daily mean value is observed, on an average, about ten o’clock in the 
morning and nine o’clock in the evening. 
On taking the mean of the hourly variations of the barometer for every 
month, it will be found that in June, July, and August, the barometer is 
generally higher in the morning and lower in the afternoon than the mean 
annual temperature, while the opposite is the case in October, November, 
and December. The mean monthly barometer height varies from one 
month to another, and this more conspicuously in the temperate than in the 
torrid zone; it is higher in winter than in the other seasons. The acci- 
dental, or not periodical variations, are considerably greater in winter than 
in summer, and this the more with the distance from the equator. Lines 
connecting places of equal annual variation of the barometer are called 
isobarometrical lines. ‘These are not parallel to circles of latitude, but 
ascend, for instance, northwards from the eastern coast of America 
towards Europe and Asia, diverge in the interior of the continent 
of the old world more and more from the equator, and then sink down 
again. j 
The absolute mean barometric condition of a place, like the mean tempe- 
rature, is obtained by taking the mean of as many mean annual barometric 
heights as possible. This depends not only on the ieve! above the sea, but 
also on the geographical position in longitude and latitude. For this reason 
the mean barometer height is not, as formerly supposed, the same every- 
where at the level of the sea. It increases from the equator in either 
direction, attaining its maximum between 30° and 40° of latitude, and then 
decreasing to between 60° and 70°. It appears again to ascend within the 
polar circle. From the equator to 15° of latitude, the mean height of the 
barometer at the level of the sea amounts to 337-338 Paris lines (29.65 to 
29.74 English inches), it then increases to 339 lines (29.83 inches), after- 
wards decreasing. The mean height of the barometer at the level of the 
sea also depends to a certain extent on the longitude, at equal longitudes and 
seasons being greater in the Atlantic than in the Pacific, the difference being 
on the northern hemisphere 1.3 Jines in winter and 1.8 lines in summer, and 
in the southern 0.3 lines in summer and 1.6 lines in winter. The following 
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