VAPOUR 17 



of vapour there is in the air, and the amount of wind moving. 

 By placing dry- and wet-bulb thermometers side by side in 

 a Stevenson screen, the effect of the wind will be greatly 

 reduced, and may then be neglected. And so we have left 

 for consideration the difference of the readings of the dry- and 

 wet-bulb thermometers and the amount of vapour. 



Now, from a long series of experiments, it has been found 

 that if the difference between the dry- and wet-bulb read- 

 ings be multiplied by certain factors, or numbers, we shall 

 obtain the difference between the temperature of the dry bulb 

 and the temperature of the dew-point, which thus becomes 

 known. Glaisher's experiments include observations made in 

 different climates, and during balloon ascents. By means of 

 Daniel's hygrometer, it has been found that they are equally 

 applicable to the hot plains in Jamaica and to the Blue 

 Mountain Peak. Glaisher's factors are given in the third 

 column in Table VIII. 



In practice, however, it will be convenient to subtract 

 unity from these factors ; and then after multiplying the 

 difference between the dry and wet bulbs by the diminished 

 factor, we get the difference between the wet bulb and the 

 dew-point. As an example, suppose that the readings of the 

 dry and wet bulbs are 85^ and 75 respectively ; we must 

 multiply their difference 10 by 1*65, the factor corresponding 

 to 85 ; this gives us 16*5, the difference between the dry 

 bulb and the dew-point ; hence the dew-point is 68*5. Or, 

 by diminishing the factor by unity, we must multiply 10 by 

 0'65, and subtract the product from 75, which gives the dew- 

 point as before. 



In this way tables can be prepared which give the dew- 

 point by mere inspection. Table IX. will be found generally 

 useful in Jamaica. 



The diurnal variation of the dew-point in Kingston is 

 small 



7am fi9^*1 



O U UJ, ,,, ,,, 4-j\/ 



XX U IXJ # \JU tj 



giving an average, as already stated, of about 70"3. 



The annual variation of the dew-point is fairly large, and 

 closely follows the minimum temperature ; this is show^ in 



A 3 



