ON AQUEOUS AND IGNEOUS METEORS. 551 



1780. Saussure, Voyages dans les Alpes, 4 vols. 4to, Neuchatel, 1796. . . Dalton's 

 Essays, 1793. Forster on Atmospheric Phenomena, 1823. Schouw, Beitrage zur 

 Vergleichenden Klimatologie, Copenhag. 1827; Ed. Jour, of Science, viii.311. Bailly 

 de Merlieux ; Resume complet de Meteor. 32mo, 1830. Rigaud. Harvey's Meteor, 

 in Encyc. Metrop. Kamtz, Lehrbuch der Meteor. 3 vols. 1831-6. Course of 

 Meteorology (trans.}, Lond. 1845. Howard's Climate of London, 3 vols. 1833. 

 Quetelet, Aperu Historique des Obs. de Meteorologie faites en Belgique, 4to, Brux. 

 1834. Forbes's Reports on Meteorology, Br. Ass. 1834, 1840 ; translated into 

 German, and amplified by Mahlmann, Leipz. 1836. Dove's Repertorium, 1839, 

 vol. iii. Daniell's Meteor. Essays, var. ed. Front's Bridgwater Treatise, 1834. 



Storms. Redfield, Silliman's Journal, 1831, p. 17. Reid on the Law of Storms, 

 Lond. 1833; Edin. Review, Ixviii. 406. Espy on do. Dove, Scientific Mem. 

 part ix. 



LECTURE LVIT. 



ON AQUEOUS AND IGNEOUS METEORS. 



THE phenomena originating from the evaporation of water constitute a 

 large proportion of the subjects of meteorology: they are materially 

 influenced by the diversities of climates and winds, which we have lately 

 considered ; and they appear to contribute to the electrical changes, which 

 form a principal part of luminous or igneous meteors : nor is the action of 

 water wholly unconcerned in many of the effects of subterraneous fires, 

 which have also a slight connexion with atmospherical electricity ; and it 

 has been conjectured that the only igneous meteors, which appear wholly 

 independent of any of these phenomena, may originate from volcanic 

 commotions in other worlds. 



The action of heat appears to detach continually from the surface of 

 water, and perhaps of every other liquid, and even solid, a certain quantity 

 of vapour, in the form of an invisible gas ; but when the space above the 

 liquid is already charged with as much vapour as can exist in it at the 

 actual temperature, the vapour, thus continually thrown off, either remains 

 suspended in the form of visible particles, or falls back immediately into 

 the liquid. This is the simplest mode of explaining the continuance of 

 evaporation, under the pressure of any dry gas, however dense, and its 

 apparent suppression in the presence of moist air, however rare. Some- 

 times also, when the temperature of the liquid is elevated, so that minute 

 globules either of steam or of air rise through it, some visible particles are 

 projected upwards by each globule, and continue to float in the air ; this 

 appears, however, to be an irregularity unconnected with the principal 

 process of slow evaporation. 



The quantity of vapour, which can exist in the space above any portion 

 of water, has been supposed by Deluc,* Volta,t and Dalton,^ to be wholly 

 independent of the nature, the density, or even the presence of the air or 

 gas wMeh that space contains : and we may easily imagine that the 

 smallest distance at which the particles of water, constituting vapour, can 



* Ph. Tr. 1792, p. 400. f Gren's Journal, iii. 479. 



t See Ph. Tr. 1826. 



