CHAP. 1. 10. OF CLOUDS. 35 



attempt further to illustrate these subjects by a 

 detail of some particular cases. In the conclusion 



had to the force of vapour actually existing in the atmosphere 

 at the time. For instance, if water of 59 were the subject, 

 the force of vapour of that temperature is l-60th of the force 

 at 212, and one might expect the quantity of evaporation 

 l-60th also: but if it should happen, as it sometimes does in 

 summer, that an aqueous atmosphere to that amount does 

 already exist, the evaporation, instead of being l-60th of that 

 from boiling water, would be nothing at all. On the other 

 hand, if the aqueous atmosphere were less than that, suppose 

 one half of it, corresponding to 39 of heat, then the effective 

 evaporating force would be l-120th of that from boiling 

 water: in short, the evaporating force must be universally 

 equal to that of the temperature of the water, diminished by 

 that already existing in the atmosphere. In order to find the 

 force of the aqueous atmosphere, I usually take a tall cylin- 

 drical glass jar, dry on the outside, and fill it with cold spring 

 water fresh from the well: if dew be immediately formed on 

 the outside, I pour the water out, let it stand a while to 

 increase in heat, dry the outside of the glass well with a linen 

 cloth, and then pour the water in again: this operation is to 

 be continued till dew ceases to be formed, and then the 

 temperature of the water must be observed; and opposite to 

 it in the table will be found the force of vapour in the 

 atmosphere. This must be done in the open air, or at a 

 window; because the air within is generally more humid than 

 that without. Spring water is generally about 50, and will 

 mostly answer the purpose the three hottest months in the 

 year; in other seasons an artificial cold mixture is required. 

 The accuracy of the result obtained this way I think scarcely 



D 2 



