TABLE XIV. 



In the Greenwich Magnetic and Meteorological Observations for 1842 and 1843, 

 Mr. Glaisher discussed the relation between the temperature of evaporation given by 

 the Wet-bulb Thermometer and the temperature of the Dew-Point as given by Dan- 

 iell's Hygrometer. Comparing the observations taken simultaneously every six 

 liours with the Psychrometer, and with Daniell's Dew-Point Hygrometer, and divid- 

 ing the average difference between the temperatures of the Wet and Dry bulb by 

 the average difference of the temperature of the Dew-Point and of the Air, lie ob- 

 tained the empirical factors given in the following Table. 



The observations from which they are deduced are those taken at the Observatory 

 in the years 1841 to 1845, for the temperatures below 35° F., and in the years 1841 

 to 1843, for the temperatures above 35° F. 



The observations made at Toronto Observatory, Canada West, in similar circum- 

 stances, in the years 1840 to 1842, were also compared in the same manner, and 

 the factors derived from them showed a very close accordance for temperatures 

 above 30° F., but were found smaller at temperatures below 30° F. 



The errors in the temperature of the Dew-Point, which may result by using the 

 Greenwich factors, though frequently within half a degree, often amount, however, 

 to ± 2 or 3 degrees, and, in extreme cases, to ± 4 or 5 degrees, as shown in the 

 volume of the Greenwich Observations for 1842, p. 60 of the Abstracts. 



Use of the Table. 



Multiply the difference between the Wet-bulb and Dry-bulb Thermometers by the 

 factor standing in the Table opposite the reading of the Dry-bulb, and subtract the 

 product from the reading of the Dry-bulb; the remainder will be the temperature of 

 the Dew-Point. 



Example. — Dry -bulb = 62° F. ; Wet-bulb = 55° ; Difference = 7°. 



Opposite 62°, in the first column, stands the factor 1.7, which multiplied by 7°, the 

 difference, gives 11°.9, to be subtracted from the Dry-bulb ; or 62° — 11°.9 == 50°. 1, 

 temperature of the Dew-Point. 



XIV. 



FACTORS TO FIND OUT THE TEMPERATURE OF THE DEW-POINT FROM THE 

 READINGS OF THE PSYCHROMETER. GlAISHER. 



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