14 Skum Lemström. 



As the air is saturated at 0° wlien tlie pressure amounts to 4,6 mm we 

 find that during tliose montlis wlien the frost is most dangerous, it may 

 fall 2 mm in May and September and 3 mm to 5 mm in all the other 

 months, before the dreaded state is reached when the dew-point is below 0°. 

 In a country so rieh in lakes as Finland this dreadful state may be rare. 



As soon as the dew-point, is reached the evaporation ceases. In most 

 cases this probal)l3^ arrives long before the température attains 0° and instead 

 of an absorption of heat by producing vapour, a soiu'ce of heat is now crea- 

 ted by condensation. 



By every kg. of vapour produced, following quantifies of heat is absorbed 



60G,5 CaP. at. 0° 

 599,6 „ „ 10° 

 592,1 „ „ 20« 



and the same (luantities are released by the opposite process, i. e. its turning 

 to water. 



Let US examine the quantity of heat 1. m^ receives from this source under 

 average circumstances and we get the following calculation: 



The température near the ground is supposed to fall as low as 



-6° 



and the tension of aq. vapour to correspond with the average worth. In this 

 case we shall have —2" at a height of 1 m. and also an average tempei'atui'e of 



~4» 



in the layer and a tension of aq. vapoiir corresponding to F=3,i mm. 

 Accepting 0",o as the dew-point the worth of F becomes 



8,0 mm 



by F=3,i mm. we have 3,5 gr. water in every m" 



„ „ = 8,0 mm. 8,3 „ 



Thus the différence or 



0,0048 kg 



bas been Condensed info water and set free 

 2,9 Cal. 



' Cal. signifies tbe qiiantity af heat necessary to rise the tenip. of 1 kilogram water from 0° to 1". 



