128 MISCELLANEOUS ARTICLES USED IN HORTICULTURE. 



Temperature of the Dew-point. If a mass of air be gradually cooled 

 it will descend to a degree of temperature at which it will be satu- 

 rated by the quantity of vapour then mixed with it. This tempe- 

 rature is called the dew-point. It can be found directly from observa- 

 tion, by the use of either Daniell's or Regnault's hygrometer. For cal- 

 culating the dew-point from observations of the dry and wet bulb 

 thermometers, we subjoin the following table of factors by which it is 

 necessary to multiply the excess of the reading of the dry thermometer 

 over that of the wet, to give the excess of the temperature of the air 

 above that of the dew-point, for every degree of temperature from 

 10 to 100. 



The numbers in this table have been found from the combination of 

 all the simultaneous observations of the dry and wet bulb thermo- 

 meters with Daniell's hygrometer, taken at the Royal Observatory, 

 Greenwich, from the year 1841 to 1854, with some observations taken 

 at high temperature in India, and others at low and medium tem- 

 peratures at Toronto. The results at the same temperature were found 

 to be alike at these different places, and therefore the factors may be 

 considered as of general application. 



