288 Mr. T. Tate's Experimental Researches on 



were saturated with moisture and suspended in a quiescent 

 atmosphere. After the lapse of two hours the single thickness 

 of calico had lost 41'6 grains of moisture by evaporation, whilst 

 the double thickness had lost 42 grains. 



In this respect spontaneous evaporation differs entirely from 

 spontaneous absorption, which, as we have seen, is dependent 

 (within certain limits) upon the thickness or weight of the mate- 

 rial, and not upon the extent of its surface. 



2. The rate of evaporation from different substances mainly 

 depends upon the roughness of, or inequalities on, their surfaces, 

 the evaporation going on most rapidly from the roughest or most 

 uneven surfaces ; in fact, the best radiators of heat are the best 

 vaporizers of moisture. 



Woollen cloth, calico, unsized paper, bran, and fine sand, of 

 equal surfaces, are very nearly the same as regards the rate at 

 which moisture is evaporated from them. Calico is a better 

 vaporizer than flannel, and very much better than water. The 

 mean result of a considerable number of experiments performed 

 in a quiescent atmosphere was, that the evaporation from the 

 surface of still water is nearly four-fifths of the evaporation from 

 an equal surface of calico. 



3. The evaporation from equal surfaces composed of the same 

 material is the same, or very nearly the same, in a quiescent 

 atmosphere, whatever may be the inclination of the surfaces. 



Equal pieces of woollen cloth, each containing 25 square 

 inches, were cemented to thin tin plates, and after being satu- 

 rated with moisture and weighed, one plate was placed horizon- 

 tally with its damp face upwards, another plate was placed upon 

 upright rods with its damp face downwards : in the same time 

 the loss from evaporation in both cases was the same, or very 

 nearly the same. 



Thus, at a summer temperature, when the air was somewhat 

 humid, the evaporation per hour from the damp surface placed 

 upwards was 4*78 grains, whilst the evaporation per hour from 

 the damp surface placed downwards was 4*71 grains; and when 

 the air was unusually dry, the evaporation from the former was 

 6*6 grains, and from the latter 6*5 grains. 



Again, at a winter temperature, the evaporation per hour from 

 the former was 2*5 grains, whilst from the latter it was 2*46 

 grains. 



The fact here enunciated is highly significant. It shows that 

 vapour is carried into the air from a damp surface for the most 

 part by the principle of diffusion, and not, as it is commonly 

 supposed, by the force of an ascensional current of vapour. If 

 the damp surface exposed to the air were higher in temperature 

 than the surrounding air, then we should most certainly have an 



