104 Dr, Masons Description 



4. Having detailed the mode by which the temperature 

 of the moistened bulb is reduced, I will proceed to state 

 the method by which the Dew-point may be obtained by 

 my Hygrometer. 



Rule. —To the difference of temperature indicated between 

 the dry and wet bulbs ; add the correction necessary 

 to produce the lowest limits of refrigeration at that 

 degree of dryness in the preceding table; multiply 

 the product by two, and subtract that result from 

 the temperature of the shade, the remainder will be 

 the Dew-point. 



^1, Dr. Mason's Hygrometer, 

 Temperature in the shade 57. 



correction. 



Example j Temperature of moistened bulb 54dif. 3 + •5=3'5 

 X 2=7 degrees of absolute dryness. 



absolute dryness. 



Temperature of shade 57 — 7=50 Dew-point. 

 L2, Dew-point Hygrometer. 



Temperature of the Dew-point ascertained by the 

 Dew-point Hygrometer above referred to 50*^ 

 Fah. 

 Temperature of shade 57. 



Dew-point ... 50 dif. 7 degrees of absolute 

 dryness on the thermometric scale. 

 I will now give a comparison between the action of my 

 Hygrometer and those of Professor Daniell and Sir John 

 Leslie. 



a. With respect to Professor DanielFs instrument, I may 

 observe, that the observation must be made in the shade, 

 or the results will be inaccurate, and a fallacious dryness 

 be indicated ; for although the Dew-point under all circum- 

 stances will be the same, the thermometer in the stem of 

 the instrument will be proportionally higher or lower, ac- 

 cording to the care taken to protect it from the different 

 sources of radiation and reflection. 



This cirumstance is particularly applicable in Madeira 

 and in tropical climates, where there is an almost impossibi- 

 lity of taking an observation in the shade, or free from the 

 influence of radiant caloric given out from the various 

 buildings. 

 In Madeira I have almost invariably found, that between 



