Evaporation in a Current of Air. 421 
The last set of experiments gave the following mean results : — 
Dry bulb 79-1° 
Dew-point 46*2 
Humidity, per cent 34 
Current velocity 4 inches from fan 1,043 ft. 
13 „ 872 
22 „ 766 
31 „ 651 
Initial temperature of water 79-8° 
Final temperature of water in still air 75*7 
6 inches from fan 71*7 
15 „ 71-1 
24 „ 71-4 
5> n 33 71*4 
Loss by evaporation in still air 16*0 gr. 
6 inches from fan 82*3 
15 „ 85-8 
24 „ 79-7 
33 „ 72-7 
Difference between the vapour pressures at the temperature 
of the water 6 inches from the fan and the dew-point 0*46 in. 
Since the loss at a distance of 15 inches is even here greater than 
at 6 inches, it is plain that the effective strength of the air current is 
again less at the nearer distance. No way that I have been able to 
devise, however, shows with any certainty that the direct current from 
the fan moves more slowly over the nearer surface than over the more 
remote. The explanation seems to be partly that the air current is not 
flowing in straight lines, but has a tumbling motion, by means of which 
subsidiary cross-currents, which have no effect on the anemometer, 
interfere more with the water 15 inches away than with the water 
6 inches away. Moreover, the anemometer necessarily integrates the 
whole of the direct current crossing the plane commanded by its vanes : 
it by no means follows that the velocity indicated by the anemometer 
is the velocity of that part of the air current that strikes the axis of 
the anemometer. This point remains for further investigation. 
The surface of the water in the gauges always becomes a good deal 
agitated by the strong draught, and in a way that certainly suggests the 
presence of cross-currents rather than a simple direct flow of air; and 
besides, the water circulates with considerable speed round the vertical 
axis of the mass of water. This agitation, conjoined with the near 
approach to equality in the temperatures of the water surfaces in the 
