126 EXCURSIONS IN MADEIRA 



The mean temperature of Funchal, according to Kirwan, is 68.9 

 of Fahrenheit, or 20.4 of the centigrade thermometer; but I 

 am inchned to think from the eighteen years observations of 

 Dr. Gourlay, a resident in Madeira, that Kirwan's informants have 

 led him to rate the mean nearly three degrees of Fahrenheit too 

 high \ as he did that of the equator ''. The difference in the 



Dr. Savignon, a Spanish physician resident at Laguna, adds to his MS. communication, 



" In a house about the centre of the city, the windows open, and a free current of 



air constantly running." The mean temperature of Santa Cruz, according to IM. von 



Buch, is 21.8 C. (71^ F.) or 4.5 C, higher than that of Laguna. I should mention, 



that the month of October of 1822 was said to be unusually warm in Madeira, where 



I arrived on the 12th: the mean of six observations at 6 a.m., was 69 F. ; of 19 ai 9 



a.m., 70.5 ; of 6 at mid-day, 73 ; of 19 at 2, 73.9 ; of 18 at sunset, 71.5 ; of 4 at 1 1 



p.m., 69.6 ; of 3 at 3 a.m., 66.1. In November, of 25 at 8 a.m., 669 ; of 18 at 2 p.m., 



67.3; of 21 at sunset, 65.8. In December, of 27 at 8 a.m., 63.6; of 28 at 2 p.m., 



64.5. In January, of 21 at 8 a.m., 61.7; of 16 at 2 p.m., 63.2; of 13 at sunset, 



62.3. Baron de Humboldt has shewn, that the half sum of the temperatures at 



sunrise, and 2 p.m., differs only some tenths of a degree from the exact daily mean; 



and that the temperature at sun-set differs only in the same small quantity from the 



mean deducted from the observations at sunrise and 2 p.m. 



" "= Humboldt, Memoircs d'Arceuil, 5. p. 512. Much error must arise from travel- 

 lers not comparing their instruments with a standard thermometer ; of five which I 

 bought at PixiJ's, M. Arago found but one exact, and in two I had to allow for an 

 error of 1°.5 R. 



