112 Mr. T. Hopkins's Observations on Malaria. 



lent in proportion to the height of the dew-point. It is how- 

 ever greatly to be lamented that we have not more particular 

 and full hygrometrical returns of the state of the atmosphere 

 in those parts of the world ; were we furnished with such re- 

 turns, there is little reason to doubt that it would be easy 

 to show, by the evidence of facts, that there is such a general 

 coincidence of a high dew-point and the prevalence of malaria 

 fever as would place them in the relation of cause and effect. 

 In the returns from the West Indies, published in the Stati- 

 stical Report of the mortality there, we have a monthly hygro- 

 metrical return for the year 1832, from the island of St. Vin- 

 cent : it is the only one in the whole report, and the substance 

 of it is o-iven at the end. It will be seen from this that the 

 mean of the dew-point for the year is 68°-86 ; the lowest, in 

 the month of February being e?"*!*, and the highest, in July, 

 70°-25. 



From the laws of evaporation as ascertained by experience, 

 it is known that the rest or motion of the air has considerable 

 influence on evaporation from wet surfaces. When vapour 

 rises from these surfaces it remains for some time resting upon 

 and near to them, where it checks further evaporation. But 

 when a current of air by its mechanical action carries away 

 the newly formed vapour, fiesh vapour immediately escapes, 

 and the process is repeated : the influence of winds in drying is 

 a familiar instance of this fact. The human body is disposed 

 to o'ive out vapour to a given extent to an atmosphere, the dew- 

 point of which is, say 70°; but without motion in the air the 

 vapour first given out would impede further evaporation ; a 

 wind removes this impediment and suffers the evaporation to 

 CTo on more freely. In accordance with this it is known that 

 malaria fevers more commonly prevail m a warm damp atmo- 

 sphere when it is stagnant ! A. brisk wind is said popularly to 

 purify the air, and to take away mephitic vapours; may it not 

 merely facilitate evaporation ? Persons suffering under mal- 

 aria fevers are relieved by fanning I Were the air charged 

 with poisonous vegetable miasmata, it would be reasonable to 

 conclude, that fanning, by bringing more of this poisoned air 

 into contact with the patient, would increase the disorder ! 

 But if the view here taken be correct, the cause of the relief 

 that is felt becomes sufficiently obvious ; the evaporation 

 which was checked by the previous accumulation of vapour is 

 accelerated by its removal. 



In temperate climates the heat of the atmosphere is consi- 

 derably below that of the human body, and the dew-point is 

 ordinarily below the temperature of the atmosphere. But 

 with reference to perspiration it seems to be as important that 



