64 



Oh the Dew Fomt. 



ience are attached to the scale of the instrument I use, — are offered 

 with ,the hope that they may induce many to make observations witli 

 that instrument. 



Wiien a smooth surface of any substance which possesses no at- 

 traction for water, is exposed to an atmosphere in contact with wa- 

 ter, if the temperature of the surface is considerably below that of 

 the atmosphere, it soon becomes covered with moisture, which in- 

 creases and assumes the form of dev/. By observing the tempera- 

 ture of this surface, we learn at what temperature the invisible vapor 

 previously existing in the atmosphere, becomes tangible in the form 

 of water ; the temperature may be considered as the " point of de- 

 position," and it bears no permanent relation to the temperature of the 

 vapori Aqueous vapor, while forming, is of the same temperature 

 as the surface of the fluid from which it is produced ; and if, after 

 this experiment we allow the surface of the substance gradually to 

 approach the temperature of the atmosphere, the moisture begins to 

 disappear and the thermometer remains stationary, while any consid- 

 erable portion of moisture is on the surface. The temperature now 

 indicated by the thermometer, is that of the vapor, and for conven- 

 ience is called the " dew point." It is evident that the point of de- 

 position bears the same relation to the dew point, as the " freezing 

 point 'f of water does to the " fusing point " of ice ; it may be below, 

 but can never be above it. By confounding these terms, some wri- 

 ters hjive caused considerable ambiguity, and we are inclined to 

 place ^ittle confidence in the results of these experiments, when we 

 are informed that " the comparison of the temperature of the air, at 

 the commencement of the experiment, with the mean of the indica- 

 tions of the thermometer, at the appearance and evanescence of the 

 dew, will give with relative accuracy the measure of the force of va- 

 por iri the atmosphere." The dew point, ascertained by the aid of 

 suitable instruments, enables us to solve several important problems, 

 whichi could not be done, when hydroscopes of animal and vegeta- 

 ble si[bstances were employed ; as these, besides being subject to 

 many imperfections, indicate a state of " dryness " when the atmos- 

 phere; is nearly saturated with moisture. 



I. Having ascertained the dew point, on referring to a table of the 

 elasticity of steam, we learn the tension of the atmospheric vapor, 

 expressed in inches and parts of the mercurial column . 



II. By dividing the numbers denoting the elasticity, by that of the 

 mean I barometric height, we ascertain the volume of vapor in one 

 hundred cubic inches of the air surrounding the instrument. 



