2 niir.osopiiY of storms. 



as a deposit of moisture on llie glass is effected, immerse tlie bulb of a 

 tliermomeler in the contents of the vessel, and, after wiping off the out- 

 side of the vessel, observe if the moisture again settles. Should it set- 

 tle, wipe off again, and continue to do so, until it scarcely collects any 

 more, being careful to stir the solution with the thermometer during the 

 observation, so that the vessel, its contents, and the bulb may acquire 

 tlie same temperature. JVuio observe — the moment that this thin film be- 

 gins to dry^ note the degree of the thermometer in the vessel, and that is 

 the temperature of the deto-point. This is the highest point on the ther- 

 mometric scale at which moisture will settle at that time, and it may 

 always be anticipated by the thin film of moisture on the vessel assum"- 

 ing all .the colors of the rainbow. The temperature of the dew-point 

 never can exceed that of the air ; sometimes it is as high, as when the 

 air is .saturated with moisture, and during rain ; but generally it is lower 

 and always diminishes in proportion to the dryness of the air. 



This method of taking the dew-point is susceptible of great preci- 

 sion. I will observe, however, that it may be obtained indirectly by 

 two thermometers as follows : — cover the bulb of one thermometer with 

 a wet rag-, then swing both briskly in the air until they become station- 

 ary ; note the difference, multiphj this by 103, and divide the product by 

 the ivet-bulh temperature, subtract tlie quotient from the dry-bulb temper- 

 ature, and the remainder icill be the dew-point. Thus : — 



Dry hull). Wet-bulb. Wet-bulb. Thry-bulb. Dew-point. 



70°_64°=6°x 103= 6 18° -^64° ^O.^-SG— 70°= 60.°344. 



This method will answer when the wet-bulb temperature ranges be- 

 tween 20° and 7o°. 



My next object 'will be to explain the method of ascertaining the 

 temperature and elevation of forming-cloud. As the temperature of 

 the dew-point is the only point at which vapor loses its gaseous form, 

 and as cloud is only vapor condensed, it follows that allowing for ele- 

 vation, the temperature of the detr-point and that of forming-cloud must 

 always be the same. Assuming the dew-point at the earth's surface to be 

 60", and the temperature of the air 70°, the temperature of the forming- 

 cloud, at its base, will, in consequence of its height, as I will soon 

 .show, be 57.°5. Now as respects the height of the base of the cloud. 

 This can also be ascertained by the thermometer, as correctly perhaps 

 as by the sextant. I have already stated that in ascending into the at- 

 rnos])here the temperature diminishes at the rate of one degree for about 

 every 3-')2 feet. It appears, however, from experiments, at least for small 

 elevations, that when air ascends, it becomes colder about 1^° for every 



