64 On the Bcu:' Foin!. 



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

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



that instrument. 



When 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 wnth moisture, which in- 

 creases and assumes the form of dew. By observing the tempera- 

 ture of this surface, we learu 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 hears no permanent relation to the temperature of the 

 vapor. 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 atmost^hcre, the moisture begins to 

 disappear and the thermometer remains stationary, while any consid- 

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

 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 " 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 have caused considerable ambiguity, and we are inclined to 

 place little confidence in the results of tliese 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, w^ill give with relative accuracy the measure of the force of va- 

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

 suitable instruments, enables us to solve several important problems, 

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

 ble substances 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 

 elasdcity 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 elasticily, by that of the - 

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

 hundred cubic inches of ihe air surrounding the instrument. 



