The great coolness, I might almost say cold, which prevails for a considerable part of the year within the tropics on the coast of Peru, causing the thermometer to sink to 12° Reaumur (59° Fahrenheit), is, as I have noticed elsewhere, by no means to be ascribed to the vicinity of the snow-covered Andes, but rather to the fogs (garua) which veil the solar disk, and to a _cold sea current_ which, commencing in the antarctic regions and coming from the south-west, strikes the coast of Chili near Valdivia and Conçeption, and thence streams rapidly along the coast to the northward, as far as Cape Pariña. On the coast, near Lima, the temperature of the Pacific is 12°.5 Reaumur (60°.2 Fahr.), whilst in the same latitude out of the current it is 21° R. (79°.2 Fahr.) It is singular that so striking a fact should have remained unnoticed until my visit to the shores of the Pacific, in October 1802. The variations of temperature of different regions depend in a great degree on the character of the bottom of the “aerial ocean,” or on the nature of the floor or base, whether land or sea, continental or oceanic, on which the atmosphere rests. Seas, often traversed by currents of warmer or colder water, (oceanic rivers), have an effect very different from that of continental masses, whether unbroken or articulated, or of islands, which latter may be regarded as shallows in the aerial ocean, and which, notwithstanding their small dimensions, exert, often to a great distance, a notable influence on the climate of the sea. In continental masses we must distinguish between sandy deserts devoid of vegetation, savannahs or grassy plains, and forest-covered districts. In Upper Egypt and in South America, Nouet in the former, and myself in the latter, found respectively at noon the temperature of the ground composed of granitic sand 54°.2 and 48°.4 Reaumur (154° and 141° Fahr.) Many careful observations in Paris have given, according to Arago, 40° and 42° Reaumur, 122° and 126°.5 Fahrenheit. (Asie Centrale, T. iii. p. 176.) The Savannahs, which between the Missouri and the Mississipi are called Prairies, and which appear in South America as the Llanos of Venezuela and the Pampas of Buenos Ayres, are covered with small monocotyledonous plants of the family of Cyperaceæ, and with grasses of which the thin pointed stalks or ears, and the delicate lanceolate leaves or blades, radiate towards the unclouded sky, and possess an extraordinary power of “emission.” Wells and Daniell (Meteor. Essays, 1827, p. 230 and 278) have even seen in our latitude, where the atmosphere has so much less transparency, the thermometer sink 6°.5 or 8° of Reaumur (14°.5 or 18° Fahrenheit), on being placed on the grass. Melloni, in a memoir, “Sull abassamento di temperatura durante le notti placide e serene,” 1847, p. 47 and 53, has shewn how in a calm state of the atmosphere, which is a necessary condition of strong radiation and of the formation of dew, the cooling