150 PHILOSOPHY OF STORMS. 







This complement being multiplied by 100 will give the 

 height of the base of the clouds in question in yards, nearly. 



2d. Put a thin, white, wet rag round the bulb and lower 

 end of the thermometer, and swing it briskly in the air, 

 and ascertain the greatest distance or number of de- 

 grees it will sink below the temperature of the air ; this 

 number is called the complement of the wet bulb : now, 

 10300 times this complement being divided by the wet- 

 bulb temperature, will give the height of the base of the 

 same clouds in yards, nearly. 1 



When the difference of the temperature of the air and 

 that of the dew point amounts to 20 or 21, or if the sur- 

 face of the ground is colder than the air, it is believed 

 these clouds hardly appear, or, if they do appear, are very 



1 When the temperature of the air is near the freezing point, great care 

 must be taken that the bulb of the thermometer be covered either with a film 

 of ice or snow entirely frozen, or wet rag entirely unfrozen ; for if the water 

 of the wet rag is freezing, or the film of ice thawing, t.he true wet bulb tem- 

 perature cannot be obtained. When the temperature of the air is a little above 

 the freezing point, and the dew point greatly below it, the wet bulb may 

 sometimes be obtained without freezing as low as 27, or 5 below the freezing 

 point ; and, in such cases, I have found that the wet bulb was exactly at the 

 game temperature as the bulb surrounded with a film of ice, allowance being 

 made for the difference of the thermometers. From this experiment it ap- 

 pears that about one seventh less evaporation goes on from ice than from wa- 

 ter at the same temperature ; for the caloric of fluidity of water is about one 

 seventh of the caloric of elasticity of steam. 



Those of our correspondents who live in very cold localities, have it in their 

 power to perform a very interesting experiment, which I recommend to their 

 special attention. From it the lowest degree at which vapor can exist may 

 be nearly determined. 



When the temperature is very low, examine how near two thermometers 

 agree ; dip one of them in water, and draw it out immediately ; it will be sur- 

 rounded with a film of ice ; place it in the same locality, and note whether it 

 sinks lower than the other : if not, no vapor rises from the ice, and, there- 

 fore, if the air is clear at the lime, vapor does not exist in the atmosphere at 

 that temperature. All experiments with the thermometer, at such low tem- 

 peratures particularly, require great care the radiation from the body, and 

 the influence of the breath, should be guarded against as much as possible. 



