Hovi- to Foretell Frosts. 



:i43 



attraction. The muslin, or material used, should be free from all 

 foreif^n matter, such as any mineral salts that niay be used in its 

 manufacture, at»d also starch. It is, therefore, wise to have it well 

 washed before usinfij. The fabric should also be often washed or 

 renewed, otherwise dust particles are apt to accumulate and form an 



incrustratioii. Touching it with 

 soiled fingers should also be avoided, 

 as the peispiration interferes to 

 some extent with the capillary 

 action. Tlie water used should be 

 pure — distilled, if possible — but 

 clean rain water will answer very 

 well. 



The dry-bulb must be actually 

 dry, for if it becomes damp, the 

 record it gives is too near that of 

 the wet-bulb instrument to render 

 its readings of value for our pur- 

 pose. In moist weather, therefore, 

 it shotdd be carefully dried bafore 

 its temperature is taken. 



There are several dew-point 

 tables in existence. I have used one 

 issued by the Washington War 

 Office, and adopted by Cahfornian 

 vignerons. Through the kindness 

 and courtesy of Mr. Baracchi, 

 Government Astronomer, to whom 

 I desire to acknowledge my 

 obligations, i am enabled to re- 

 produce the dew-point table 

 of the Koyal Meteorological 

 Society. This is uniforndy 

 adopted in all British 

 observatories. The various 

 tables differ slightly, and it 

 must be understood that 

 the values are approximate 

 only. But the work of foretelling frosts is of sufficient importance, 

 and the trifling trouble incurred by the observer has been amply 

 compensated by the value of the crop saved by being forewarned. 



Dew, as the merest tyro knows, is formed on calm, clear nights 

 only. If the sky becomes overcast with clouds they prevent the 

 radiation of heat into space, and the air is not cooled sufficiently for 

 the deposition of dew. The prevalence of wind keeps constantly 

 renewing the various strata of the atmosphere, and the air in motion 

 near the earth's surface prevents cooling to the dew-point temperature. 

 The dew-point temperature is that temperature at which dew is 



Dry and Wet Bulb Theimometers 



