310 On Malaria. 



daily added to the quantity and concentration of the miasmata, the 

 cooler atmosphere of autumn keeps it perpetually condensed, and it 

 is not only more virulent, but the exposure to it is continual through- 

 out the day. 



Neither does the sickly season except in tropical regions com- 

 mence during the prevalence of rain. Vegetation is then flourishing, 

 the rain absorbs and dilutes any poison which may be exhaled, and 

 when the clouds subside, the sun dries up the moisture. As the sea- 

 son advances the quantity of water diminishes, the herbage which 

 had been macerated is then exposed to the heat, the poison increases 

 in deadly energy by concentration, and the exhalations hover nearer 

 the surface. Hence another reason why fevers and epidemic sick- 

 ness prevail after the heats and rains of summer are past. 



Another distinctive property of malaria is its specific gravity,^ 

 which being greater than atmospheric air, prevents its effects from being 

 felt above a certain altitude. Dr. Ferguson states that " a hill in An- 

 tigua, six hundred feet high, is exempt from its effects, while the 

 country is replete with it at its base." M. de Rigaud de Lisle " estab- 

 lishes the height of safety near Rome, at from six hundred and eighty 

 two, to one thousand feet." Sezza, nine hundred feet above the Pon- 

 tine marsh, is free from disease. " Erceero, nine hundred yards 

 above La Vera Cruz, is exempt from the fevers of the lower land."-]- 



It is difficult to ascertain satisfactorily why it sometimes attacks 

 one side of a street, and leaves the other unhurt ; sometimes cuts 

 off a family or a neighborhood, while those in the immediate vicui- 

 ity escape; and even, when transported from a distance, invades 

 certain places perennially through successive years, and avoids oth- 

 ers which appear equally exposed. A remarkable instance cited by 

 Dr. McCulloch is " on the road between Chatham and Brighton in 

 England, where the ague affects every town and single house on the 

 left hand side of the turnpike, and does not touch the right side, 

 though the road itself forms the only line of separation." It seems 

 probable however, that there being an attraction between the miasma, 

 and the identical moisture with which it is exhaled, it does not equal- 

 ly pervade the atmosphere, but is winged about in flaws and 



* This would appear to be generally but not universally true. We are credibly 

 informed, that in the marshy regions of Maryland and of some other portions of the 

 Southern States, those who live at the A'ery edge of the marshes escape, while those 

 on the higher grounds are assailed by fever. — Ed. 



\ Humboldt. 



