Mr. T. Hopkins's Ohservatioiis on Malaria. 117 



perature. But still the dew-point in Captain Hall's ship 

 must evidently have been much below what it was in the Car- 

 narvon. The above account of Captain Hall furnishes an 

 instance of what has been already stated in a general form, 

 that the heat of the sun rarefies the air, and makes it flow 

 over high in the atmosphere, before it becomes fully saturated 

 with vapour. The most important exceptions to this general 

 law are to be found in those places where malaria most 

 abounds, that is in heated valleys or marshes near to, or within 

 the tropics. The sea at its surface, between the tropics, is 

 seldom found much hotter than 80°; but we have seen that in 

 African valleys the temperature has been from 100^ to 118°, 

 and it follows that the latter places are likely to have a much 

 higher dew-point than the former. In some of the African 

 rivers, commanders on losing many of their men and having 

 others ill of fever, have put out to sea in order to get out of 

 reach of the poisonous influence ; this however it is clear, from 

 what has been said, may have been only a case where an at- 

 mosphere of, say 100°, highly charged with steam, was ex- 

 changed for one of 80°, with a dew-point of perhaps only 70° 

 or 65°. The superabundance of steam from heated valleys 

 or flat coasts may however, it is obvious, be taken far out to 

 sea by gentle winds, and thus parts over the sea may be 

 made as unhealthy as hot and damp valleys. This may have 

 been the case with the Carnarvon on board of which were the 

 Landers. But so little attention has been paid to hygrometry 

 by navigators, as to leave us in ignorance of what the dew- 

 points really were. 



There seems good reason to believe that a warm and moist 

 air may be rendered more healthful by heating it, because 

 then the temperature would be so much higher than the dew- 

 point as to render it a drying air. But It is still more certain 

 that a reduction of the dew-point much below the temperature 

 removes the cause of malaria fevers : " The Harmattan," says 

 Lander, " a land wind, passes over the sands of Africa, and 

 while it lasts the dryness of the atmosphere produces an un- 

 pleasant feeling, but it is said to be not injurious to health." 

 And again, " The effects of the Harmattan after the rainy 

 season are most beneficial in drying up the vapours with 

 which the atmosphere is loaded ; and it has been obsei'ved, 

 that on the return of this wind at die end of the rainy season, 

 the recovery of invalids commences." But this dry wind 

 seldom continues longer than three or four days. What the 

 state of the dew-point is while it lasts I have never learned, 

 but it must be very low compared with tiie temj)erature, as 

 it dries the country with extiaordinary rapidity. 



