142 Baron Humboldt on the Mean Temperature 



were made with an excellent thermometer for maxima, by 

 Six. The mean temperature of Benares is 77° 36. * 



The extreme heat which occurs in the southern portion of 

 the temperate zone, between Egypt, Arabia, and the Gulf of 

 Persia, is the simultaneous effect of the configuration of the 

 surrounding lands, of the state of the surface, of the constant 

 transparency of the air deprived of aqueous vapours, and the 

 length of the days, which increase with the latitudes. Between 

 the tropics, even great heats are rare, and generally do not 

 exceed at Bombay 91° and at Vera Cruz 95°. It is almost 

 needless to state, that in this note we have referred only to ob- 

 servations made in the shade, and far from the reflection of 

 the ground. At the equator, where the two solstitial heights 

 reach 66° &W, the times of the sun's passing the zenith are 

 distant from one another 186 days. At Cumana, the height 

 at the summer solstice is 76° 59, and that of the winter sol- 

 stice 56° 5f 9 and the times of passing the zenith, 17th April 

 and 26th August, are distant 131 days. Farther to the north, 

 at the Havanna, we find the solstitial height in summer 

 89° 41', and of winter 43° 23 7 , and the distance of the passage 

 (12th June and 1st July) 19 days. If these passages are not 

 recognized with the same evidence in the curve of the month, 

 it is because their influence is marked in some places by the 

 occurrence of the rainy season, and other electrical phenomena. 

 The sun is at Cumana during 109 days, or more exactly dur- 

 ing 1275 hours, (from the 28th October till the 14th Febru- 

 ary,) lower than under the equator, but in this interval its 

 maximum of zenith distance does not exceed 33°.55. The 

 retardation in the sun's progress, in approaching the tropics, 

 increases the heat of places situated farther from the equator, 

 particularly towards the confines of the torrid and temperate 

 zones. Near the tropics, for example at the Havanna, lat. 

 23° 9 / , the sun employs twenty-four days to describe a degree 

 on each side of the zenith ; under the equator it requires on- 



* Mr Prinsep makes the mean temperature of Benares in 



1822 76°.81 



1823 76 .40 



and the greatest range from 1 1 H° to 45° ; the mean heat of a well thirty- six 

 feet deep was 79°.71 c .— -Ed. 



