16 THE METEOROLOGY OF JAMAICA 



the amount will be decreased, the vapour will be condensed,. 

 and form minute drops of w^ater, which adhere to the 

 sides of the jar, or trickle down to join the water at the 

 bottom. 



In this way dew is formed. At night the temperature of 

 the grass, leaves of trees, roofs of houses, etc., is reduced by 

 radiation ; the air near them is chilled, and it cannot then 

 contain all the vapour it previously sustained ; the excess is 

 quietly and gradually deposited in the form of dew. In the 

 country the amount of dew is so large that it drips from the 

 eaves of the houses like rain. In Kingston the air is much 

 drier, and there is but little dew. 



In this way, again, clouds are formed and rain produced. 

 But we have said enough to show the importance of aqueous 

 vapour, and we must now return to its measurement. 



If from an elaborate series of experiments, we knew the 

 tension of saturated vapour for each degree of temperature, we 

 could ascertain the amount of vapour present in the air at 

 any given time and place by noting the temperature of the 

 dew-2)oint, or the temperature at which dew begins to form. 



Such experiments have often been made, but the best 

 were conducted by Eegnault, the French chemist ; the results 

 are given in the second column in Table VIII. 



In order, therefore, to find the amount of vapour present 

 in the air at any time, it is only necessary to find the dew- 

 point and to employ Table VIII. Thus, if the average tem- 

 perature of the dew-point is 70^*3 in Kingston, the aqueous 

 vapour in the air there exerts an average pressure or tension 

 of 0*741 inch. 



But in order to find the dew-point, it is generally necessary 

 to reduce the temperature. This may be done by ether; and 

 Daniel's hygrometer is adapted to thedirect observation of the 

 dew-point. 



Such observations, however, would be tedious, and the 

 dry- and wet-bulb hygrometer is used in preference. If the 

 bulb of a thermometer be wrapped in some very thin muslin 

 and kept continually damp by means of a connecting thread 

 which dips into a small cistern of water, the thermometer 

 will show the temperature of evaporation, which depends on 

 the rate of evaporation, which again depends on the amount 



