80 PHILOSOPHY OF STORMS. 



" dew point" is, to use a thin tumbler of tin, kept very 

 bright and clean on the outside and in the summer cold 

 water, and in the winter snow or ice, and if necessary salt, 

 mingled with water and when these are not at hand, a 

 mixture of muriate of ammonia and nitrate of potash, in 

 equal quantities, pounded very fine, put into the tumbler 

 with water. By any of these means a temperature may 

 soon be obtained below the " dew point." When dew set- 

 tles on the tumbler it must be carefully wiped off, very dry, 

 and the fluid within stirred with a thermometer and this 

 must be repeated until the fluid is gradually heated up by 

 the air, so that the moisture ceases to settle : the highest 

 temperature at which it will settle is the "dew point," 



For observations of the dew point to be of any value, 

 however, they must be made constantly, every day at least 

 once a day. 



Again, some may be unwilling to take the dew point, 

 who would be glad to know it may be obtained, approxi- 

 mately, by the following indirect method : 



Take two thermometers that agree, or allow for the dif- 

 ference cover one of them with a wet white rag, and 

 swing them simultaneously in the air, (for it will not do to 

 let them be at rest, unless the wind is blowing fresh) ; when 

 it is discovered that they cease to change by swinging, take 

 103 times their difference, and divide it by the wet bulb 

 temperature, and subtract the quotient from the tempera- 

 ture of the naked bulb the remainder will be the dew 

 point. This formula is founded on experiments from 20 

 Fahr. to 80, and does not differ, at either extreme, from 

 the most careful experiments. We cannot refrain from 

 saying, we are sure that every lover of the science will be 

 richly rewarded for all the pains he may bestow on the dew 

 point, even independent of the results which will undoubt- 

 edly be derived from a comparison of these simultaneous 

 observations. 



