132 EXCURSIONS IN MADEIRA 



winds at first, and of south and south-west, sometimes amounting 

 to gales, afterwards. Thus, although situated within the tem- 

 perate zone, and therefore subjected to a far greater number of 

 perturbing causes, yet from the vicinity of Madeira to the tropic, 

 we are enabled to recognise the influence of the same laws which 

 regulate the setting in of this season in the regions of the torrid 

 zone. It has been submitted by one of the first authorities on 

 these subjects, that while the north-east breeze prevails, it prevents 

 the air, which reposes on the equinoctial seas and regions, from 

 being saturated with humidity ; the ascending current of heated 

 and humid air being regularly replaced below, by dryer and cooler 

 currents, from the north : but when this breeze ceases, the 

 columns of air are no longer displaced or renewed, and, conse- 

 quently, the humidity is accumulated to saturation. The north- 

 east breeze being created by the difference of temperature between 

 adjoining regions, abates of course in proportion as that difference 

 of temperature diminishes : now the month in which the tempera- 

 ture of Madeira differs least from that of the region or band of 50" 

 N, is September, at the end of which the first rains and ^vesterly 

 winds generally occur. It does not appear to me, however, that 

 the rainy season of the northern equinoctial regions ought to occur 

 at the time of the sun's passing the zenith of the different places, 

 as De Humboldt considers ; for surely there will be the least 

 difference between the temperature of the northern and the equa- 

 torial regions when the sun is nearest to the former, and the most 

 distant in northern dechnation from the latter. Accordingly, 

 we find that the rains commence at Cape Coast, and Sierra Leone, 

 not in the beginning of April or September, when the sun passes 

 the zenith of these places, but towards the end of June, when it 

 has reached the northern tropic. Being but 150 feet above the 

 sea, when I made my observations, the lower regions of the air 

 were so slightly charged with electricity, that I could not discover 



