<508 PHYSICAL GEOGRAPHY. 



3. The humidity of the atmosphere. The quantitative 

 relations of the humidity depend on the proportion between 

 the terrestrial and oceanic surfaces ; on the distance from the 

 Equator, and the height above the sea ; on the form in which 

 the aqueous vapour in the atmosphere is precipitated; 

 and on the connection of these precipitations with changes 

 of temperature and with the direction and succession of 

 winds. 



4. The electric tension of the atmosphere ; of which 

 the primary source, when the sky is serene, is still much 

 contested. Under this head we have to consider the relation 

 of ascending vapour to the electric charge and form of 

 clouds at different periods of the day and year ; the influence 

 of cold and warm zones of the earth, and of low or elevated 

 plains; the frequency or infrequency of thunder-storms, 

 their periodicity, and their formation in summer and winter ; 

 and the causal connection of electricity with the exceedingly 

 rare occurrence of night hail, and with the phsenomena of 

 water-spouts and of columns of sand, investigated with much 

 ingenuity by Peltier. 



The horary variations of the barometer, which under the 

 tropics present two maxima (at 9 or 9J A.M., and at 

 10J or lOf P.M.) and two minima (at 4 or 4J A.M. 

 and 4 A.M., which are nearly the hottest and the coldest 

 hours of the day), were long the object of my most careful 

 daily and nightly observation. ( 382 ) Their regularity is such, 

 that, in the day time especially, we may infer the hour from 

 the height of the column of mercury, without being in error 

 on an average more than fifteen or seventeen minutes. In 

 the torrid zone of the New Continent, I have found the regu- 



