52 



PHILOSOPHY OP STORMS. NO. V. ' 



BY PROF. W. L. ATLEE, M, D, PHILAPELPHIA, PA. 



From what has heretofore been said, it will now be readily undei'= 

 stood that whenever the temperature of air is reduced down to, or be- 

 low its dew-point, the condensation of its vapor into water must be 

 the result. This condensation, too, is just in proportion to the reduc- 

 tion of temperature, and is calculated in the same way as the quantity 

 and tension of the vapor. If, for instance, a certain bulk of air, say 

 the air in a room, could, by any circumstance, be suddenly reduced be- 

 Jow 60 degrees, the assumed dew-point, a portion of the vapor would 

 be condensed into water, and a cloud or fog would be the result. If 

 the thermometer would fall 20 degrees below the dew-point, one-half 

 of the vapor in the room would be reduced into water ; if it would 

 fall more than twenty degrees, more than half the vapor*, if less than 

 20 degrees, less than half; and thus the deposition would vary with the 

 temperature. This was beautifully illustrated by Professor Espy, by 

 means of an instrument called tlie Nephelescope. This instrument con- 

 sists of a bottle or glass receiver, having a stop-cock fastened into its 

 neck, and also a barometer-guage, resembling an inverted syphon, com- 

 municating with its cavity. By means of a condensing syringe, Prof. 

 Espy condensed an equal bulk of air into the receiver, and the mercury 

 in the syphon-guage was observed to rise. After the equilibrium of tem- 

 perature which had been destroyed by this operation, had been restored, 

 he applied a measure carefully to the barometer-guage to ascertain how 

 much higlier the mercury stood in the outer leg than in the inner, and 

 then turning the cock, he again permitted the air to escape, and the mer- 

 cury was seen to fall suddenly to its original level, lie now quickly 

 turned the cock again, cutting ofl' all communication with the external 

 air, and the mercury began to rise again and remained up, because the 

 air within received heat from without, and the difference of level being 

 measured as before, this indicated the number of degrees cooled by a 

 given expansion. In this experiment, the air, condensed within the re- 

 ceiver, was permitted to escape, while the remaining half, by its clastic 

 force, expanded and filled the receiver. In consequence of this expan- 

 sion a great deal of cold was produced, and this rapidly condensed the 

 vapor in the air into a dense cloud, which was plainly seen from every 

 part of the room. At the moment the air flies out of the receiver tlie 

 air within expands, and the amount of this expansion is indicated by 

 the extent of the depression in the guage, while its subsequent rise en- 

 ables us to calculate tlie exact amount of cold produced by the expan- 

 sion, and also the amount of vapor condensed by the cold. Another 



