110 Mr. T. Hopkins's Obsetvations on Malaria. 



chiefly at Manchester, he found that in the months of June, 

 July, and August the dew-point generally ranged from 50° to 

 60°; that it was only once at 64°,'once at 63°, five times at 62°, 

 three times at 61°, and twenty times at 60°. But in other parts 

 of the world where malaria prevails, different meteorological 

 facts present themselves, and furnisli ground for presuming 

 that the laws of evaporation aftect the people of those coun- 

 tries differently to what they do the inhabitants of our colder 

 climate. In Rome, in the hottest month of the year, du- 

 ring the day the temperature ranges from 90° to 100° in the 

 shade, and the air is damp ; the dew-point must therefore be 

 high. 



In Captain Alexander's observations on the western coast 

 of Africa, we are told that " four days after leaving Teneriffe, 

 while proceeding for the river Gambia, on the 6th of October, 

 the wind at south-east swept over the ocean charged with 

 moisture. At noon the thermometer under the awning was 

 at 80°, while with Daniell's hygrometer I found the dew-point 

 at 70°. On the 7lh of October during a sirocco the thermo- 

 meter was at 86°, the hygrometer 76°. At the end of Novem- 

 ber in the Bight of Benin, in sight of the island of St. Thomas, 

 the temperature was 84°, the hygrometer 79° !" In the me- 

 teorological register kept by Mr. Oldfield, surgeon in Laird 

 and Oldfield's expedition up the Niger, we find it staled that 

 during the month of April, on the river, the temperature was 

 generally above 100°, and on the 14th of April it reached 

 118°. No hygrometrical return is given, but as the air is 

 stated to have been damp, it may safely be inferred that the 

 dew-point was extremely high. 



Here then we have instances where the dew-point was in 

 different places at the respective heights of 42°, 46°, 53°, 60°, 

 70°, 76°, and 79°, and from Oldfield's register it may be pre- 

 sumed to have been much higher on the Niger, probably 90°. 

 Suppose water of the temperature of 98° to be placed in these 

 various atmospheres, and it will be seen that the energy of 

 evaporation of this water would be very different in those dif- 

 ferent places. Evaporation would go on much more freely 

 when the dew-point was at 42° than when at 60°, at 60° than 

 when it was at 70°, at 70° than 76° or 79°; and the nearer the 

 dew-point approached to 98°, the temperature of the water, the 

 more feeble would be the evaporation. Now the temperature 

 of the human body in its healthy state being 98°, wlien this 

 body is placed in an atmosphere the dew-point of which is 

 42° or even 60°, evaporation will go on vigorously. Let the 

 dew-point rise to 70°, and still evaporation might possibly 

 go on with considerable energy ; but should the dew-point 



