of air. A rule which gives fairly accurate results when the tempera- 

 ture is at 50 degrees or below is to multiply the difEereJice between the 

 reading of the wet and dry bulb by 2.5 and subtract the product from 

 the reading of the dry bulb. The number so obtained is the approx- 

 imate degree to which the temperature will fall during the night 

 rmless the wind changes and blows from a moister quarter, or cloudi- 

 ness intervenes. This is also the dew point. On a fine, clear day 

 toward evening or after sunset, suppose we find the dry bulb reads 49 

 and the wet bulb 39. Then 10 times 2. r,=25; 49— 25=24. You may 

 be sure that a frost will occur. If the readings are 49 and 45, we have 

 4,-2.5=10; 49 — 10=39, and no frost will occur. If the readings are 

 ■19 and 42, we have 7x2.5=17.5; 49—17.5=31.5; a light frost will 

 probably occur and tender plants should be protected. By the use of 

 dew-point tables published by the Weather Bureau somewhat more 

 exact results may be obtained. But either method gives results that 

 may be of great ^';llTle in protecting perishable crops. 



lY. 



THE WORK OF THE WKATHER BUEEAT 



The United States has the largest and best weather service in the 

 world. Observations are fallen at 8 a. m. and 8 p. m., Washington 

 time, at over 100 regular and at several hundred volunteer stations. 

 The observations taken furnish climatic records of very great value 

 and are at once utilized for the purpose of predictinn' the weather con- 

 ditions for the 36 hours following earh nljser\nlion, and issuing storm 

 and frost warnings when necessary. 



A description of the instruments and the methods of using them 

 may be found in a pamphlet called "Instructions to A^olunteer Ob- 

 servers," issued by the AVeather Bureau. 



The lesson for us is that the present magnificent system which 

 saves thousands of li\'es and many million dollars worth of property 

 every year is the outgrowth of careful observation of local phenomena. 

 The foundation of our knowledge of American storms is said to be 

 the observation of Franklin, that the severe coast storms in which the 

 wind blew from the northeast reached Philadelphia before they did 

 Boston. A further study of the matter developed the fact that while 

 the wind moved from, the northeast the storm itself moved toward the 

 northeast. It ^\as found that the winds in such storms did not blow 

 in straight lines, but in spirals. At the common center of these there 

 was an area in which the air pressure was low and the wind circled 



