Dr Heineken on the Sirocco Winds at Funchal. 45 



than positive cloudiness. I can explain them in no better 

 manner. 



It will be seen by the foregoing tables, that the true sirocco 

 does not visit us more than twice, or at most thrice annually, — 

 that it never lasts above ihvee days — that it always blows 

 from the south of east — and that it is always remar'kahly dry. 

 (In the Mediterranean, I am told, it is as remarkable for its 

 dampness.) I have never experienced the sirocco wind ex- 

 cepting here. I have never read or heard a detailed account 

 of it elsezvhere, and I am quite ignorant of the theory of it, if 

 there be one. I suppose, however, that it is amenable to the 

 general law governing local and periodical winds, and that it 

 arises from an effort to restore an equilibrium, which has been 

 somewhere, and from some cause, disturbed. This disturbing 

 cause I should imagine to exist at the origin of the peculiar 

 wind to which it had given rise, viz. in some part of Africa, 

 where a large body of highly heated air becoming suddenly 

 condensed, an immediate rush of the surrounding denser me- 

 dium had set a quantity of the rarefied atmosphere in motion, 

 in that direction in which it had the least resistance to over- 

 come ; and that such current so produced constituted the si- 

 rocco windf varying in temperature and dryness, according to 

 the distance and medium through which it had passed, and 

 ceasing altogether as soon as it had acquired the density of 

 the surrounding atmosphere. That the point of its perfect 

 condensation is in what I have termed our true siroccos, not 

 very far to the north-west of the island, is probable from the 

 frequency of thick, imperfect, and partial siroccos, compared 

 with those which are complete ; and I have little doubt, that, 

 in accompanying the true siroccos in that direction, we should 

 have a gradation of such changes, and terminating perhaps in 

 rain. It seems to me that the reason why the Mediterra7iean 

 siroccos are damp is because they have there arrived nearly 

 at their utmost limit, and have met with a sufficient decrease 

 of temperature to induce a deposition of some of the water 

 which they held in solution — that in our ti'ue siroccos, on 

 the contrary, a much lower temperature than they meet with 

 here is required for that purpose — that in ^' our partiaV ones 

 but little lower would produce the effect — that " our imperfect'''' 



