320 COSMOS. 



much contested. Under this head we must consider the rela- 

 tion of ascending vapours to the electric charge and the form 

 of the clouds according to the different periods of the day and 

 year, the difference between the cold and warm zones of the 

 earth, or low and high lands; the frequency or rarity of 

 thunder storms, their periodicity and formation in summer 

 and winter; the causal connection of electricity with the in- 

 frequent occurrence of hail in the night, and with the pheno- 

 mena of water and sand spouts, so ably investigated by Peltier. 



The horary oscillations of the barometer which in the 

 tropics present two maxima, (viz., at 9 or 9 A.M., and 10^ or 

 lOj P.M., and two minima at 4 or 4 P.M., and 4 A.M., occur- 

 ring, therefore, in almost the hottest and coldest hours,) have 

 long been the object of my most careful diurnal and nocturnal 

 observations.* Their regularity is so great, that, in the day- 

 time especially, the hour may be ascertained from the height 

 of the mercurial column without an error, on the average, of 

 more than fifteen or seventeen minutes. In the ton-id zones 

 of the New Continent, on the coasts as well as at elevations 

 of nearly 13,000 feet above the level of the sea, where the 

 mean temperature falls to 44-6, I have found the regu- 

 larity of the ebb and flow of the aerial ocean undisturbed by 

 storms, hurricanes, rain, and earthquakes. The amount of 

 the daily oscillations diminishes from 1'32 to 0'18 French 

 lines from the equator to 70 north latitude, where Bravais 

 made very accurate observations at Bosekop.f The supposi- 

 tion, that much nearer the pole the height of the barometer is 

 really less at 10 A.M. than at 4 P.M., and consequently that 

 the maximum and minimum influences of these hours are 

 inverted, is not confirmed by Parry's observations at Port 

 Bowen (73 14'). 



The mean height of the barometer is somewhat less under 



* Observations faites pour constater la marcJie des variations 

 Jioraires du Barometre sous les Tropiques, in my Relation historique 

 du Voyage aux Regions Equinoxiales, t. iii. pp. 270-313. 



f* Bravais, in Kaemtz and Martins, Mettorologie, p. 263. At Halle 

 (51 29' N. lat.), the oscillation still amounts to 0'28 lines. It would seem 

 that a great many observations will be required, in order to obtain results 

 that can be trusted in regard to the hours of the maximum and mini- 

 mum on mountains in the temperate zone. See the observations of 

 horary variations, collected on the Paulhorn in 1832, 1841, and 1842. 

 (Martins, Meteorologie, p. 254.) 





