114- Mr. T. Hopkins's Observations on Malaria. 



thrown off by evaporation remains in the body, with the heat 

 requisite to evaporate it, and this may cause fevei*, and carry 

 the temperature of the body up to 112°. 



The evidence of what is called dampness of the atmosphere, 

 as given by writers generally, is of a loose and unsatisfactory 

 nature. Even in the valuable report from the West Indies 

 already alluded to, the intelligent compiler says : " If the 

 mortality of the troops depended materially on the influence 

 of moisture, we might expect it to attain its maximum in those 

 stations where the greatest fall of rain takes place, whereas 

 the average mortality of troops in Jamaica is at least double 

 that which prevails among those in British Guiana, though 

 the quantity of rain which falls in that island is little more 

 than one half as great as in Guiana." 



Here we see that the quantity of rain that falls is taken 

 as evidence of the moisture of the climate, whereas it is quite 

 possible that rain may fall from a considerable elevation while 

 the dew-point is comparatively low near the surface of the 

 earth, while on the other hand the dew-point may be very 

 high without much rain falling. When the autumnal rains 

 fall at Rome malaria is diminished, but those rains by cooling 

 the country lower the dew-point or reduce the quantity of 

 steam in tlie air. In the summer in Great Britain the dew- 

 point is at times 15 or 20 degrees below the temperature 

 when clouds are forming and rain falling from a considerable 

 elevation. The fall of rain is evidently not an indicator of 

 the state of the dew-point near the surface of the earth. 



In directing our attention to this subject it may be worth 

 while to observe, that it is not so much the mean temperature 

 of certain places that should be noticed, as the high tempe- 

 rature of the days vaporising much water, and thus raising 

 the dew-point very high. A temperature of 100° in the day 

 and 60° at night, making a mean of 80°, might in a marshy 

 country, such as that near Rome, give a dew-point of, say 

 90*^, at or near sunset, while a uniform temperature of the 

 mean 80° could not possibly give a dew-point of 90°, nor 

 above 80°. A day temperature of 70° and a night tempera- 

 ture of 40°, making a mean of 55°, might give a dew-point of 

 60°, but a uniform temperature of the mean could not give 

 so high a dew-point as 60°. As the former may be taken to 

 represent the Campagna of Rome, the latter may be consi- 

 dered to represent the marshes of Lincolnshire. Now when 

 on the going down of the sun the temperature of the Cam- 

 pagna sunk to 90°, the temperature and the dew-point might 

 belhe same ; and when in Lincolnshire the temperature sunk 

 from 70° to 60°, the temperature and dew-point might also be 



