Mr. T. Hopkins's Observations on Malaria. 109 



from parts nearer to the poles, and the process is repeated and 

 continued. The vapour that is taken up with the rarefied 

 air flows with it through the higher region towards the poles, 

 and becoming cooled in its passage, a portion of the vapour 

 is deposited in the form of rain. The air thus cooled and 

 deprived of part of its vapour returns from each polar region, 

 flowing along the surface of the earth, and has its temperature 

 increased by the sun's heat as it advances towards the equator ; 

 it consequently becomes a dry air, or is disposed to take up 

 vapour from moist surfaces. Thus we see that in accordance 

 with the general laws of nature, the air near to the surface of 

 the earth which is flowing from the poles to the equator is a 

 dry air. 



But through the influence of local causes this course of 

 nature, though general, is not universal. While in some parts 

 the air near the surface of the earth is very dry, in other 

 parts it is so fully charged with moisture as to prevent further 

 evaporation taking place at the existing temperature. In this 

 case, the dew-point, being that degree of the thermometer at 

 which dew is formed from the atmosphere, is the same as the 

 temperature. In different parts of the world there are various 

 degrees of dryness in the atmosphere, and these are expressed 

 by the relation which the dew-point bears to the temperature. 

 When the dew-point is only one degree below the tempera- 

 ture evaporation goes on very feebly ; when more than one, 

 it proceeds more vigorously ; when at ten, fifteen, or twenty 

 degrees below the temperature, evaporation goes on with pro- 

 portionally increased energy. Dr. Dalton made various ob- 

 servations on the subject near to and on the mountain of Hel- 

 vellyn in Cumberland. He found that at one time, in the 

 valley below the mountain, the temperature was 70", the devv- 

 pointSS"^, difference 17°; at another time the temperature was 

 56°, the dew-point 46°, difference 10°: of course the energy 

 of evaporation would in each case be proportioned to the dif- 

 ference. At the same time, on the mountain, 855 yards above 

 the valley, the temperature was the first time 56'', dew-point 

 46°, difference 10°. The second time it was 46°, devv-i)oint 

 4-2°, difference 4°. 



Thus in these four observations the energy of evaporation 

 would be as the numbers of the difference, or as 17, 10, 9, 

 and 4. In the same paper in which the above facts are to 

 be found, published in the 4th volume of tlie Society's Trans- 

 actions (Manchester) the Doctor has given a table of numbers 

 exhibiting the drying power of the atmo;sj)here, which is one 

 form of expressing the energy of evaporation. 



Dr. Dalton also states that in twenty years of observation 



