AND PORTO SANTO. 127 



mean temperature of several years scarcely ever exceeds 1° of 

 Fahrenheit in Madeira ; and the difference between the means of 

 February and August, which may be considered as the extremes 

 of heat and cold, averages 10°. The mean temperature of Lisbon, 

 according to Lieutenant-Colonel Franzini's observations for 1816 

 and 1817 •*, is 60.2 and the difference between the means of 

 January and July, 21°. 5 F. This difference in the mean tem- 

 peratures of Lisbon and Madeira gives 5°. 3 F. for 6° of latitude, or 

 9° F. for the 10° between the parallels of 30° and 40° ; being 1°.7 

 more than the allowance of Baron de Humboldt : but if we admit 

 Kirwan's mean temperature, this variation from Humboldt's con- 

 clusions will amount to an excess of 3°.7 and almost equal the 

 difference between those parallels in the new continent. 



"A Funchal la temperature des caves parait ^tre de 16°.2 C. 

 (61° F.), par consequent, de 4° C. (7°.9 F.) au dessous de la tem- 

 perature de I'air. Nous reviendrons dans la suite sur cette 

 difference remarquable entre les souterrains a I'isle de Madere et 

 I'atmosphere circonvoisine *." I think I may venture to contradict 

 this anomalous difference. There is not a subterraneous cellar, 

 or crypt, in the island : the deepest caverns, are the " Furnaces of 

 St. John," (close to Funchal on the N.W., and about 240 feet 

 above the sea) formed out of a deep bed of scorice. In the largest, 

 (128 feet to the inmost part, which is wide, spacious, and lofty, and 

 about fifteen feet below the level of the mouth) I made the fol- 

 lowing observations : November, 4th, 1 p.m., temperature of air 

 7r..5; hygrometer 51°; temperature of the inmost part of the 

 cavern, 67° ; hygrometer 731°. January 4th, 2 p.m., temperature of 

 air 66° ; just within the mouth, 64° ; inmost part, 63i°. Again, in 



•* Observacoes Meteorologica Feitas na Cidade de Lisboa, no anno de 1816 e 1817, 

 Offerecidas a' Rede Academia dos Scienceas, p. 123,727. 



• UvuBOLDT, Annates de Chimie, p. 602. Rel. Hist. p. 424, qto. I quote from my 

 MS. extracts from these expensive works, and have omitted^to note the volume. 



