24 Selim Lemström. 



l:o. We calculate the qixantity of heat necessaiy for the raising of tlie 

 température of the cooled hiyer of air to a little ahove 0°, and add to it the heat 

 discharged at the formation of ice, having calculated the quantity of dew 

 which falls from the layer of air. 



Having to deaî with the quantity of heat which is to be restituted to 

 plants at a température of somewhat above 0", Ave retain, on calculating, the 

 specific heat of the water, and as, on the occasions when the température will 

 sink below 0", it is always a question of communicating heat to liquid veget- 

 able sap, inter alia, we tliink it unnecessary to take the slight advantage of 

 using the specific heat of the ice. 



As the air contains moisture and other elementary parts, we estimate its 

 specific heat to he 



0,247. 



The size of the field we présume to be 10 hectares, the température near 

 the ground at sunrise = — 6°, the humidity of the atmospher such that the air 

 is half saturated, and further, that the average température of the day has been 

 16°, consequently 6,5"™ of pressure of aq. vapour and 0,8 gr of water to m^; 

 the dew-point being at + 5",o. 



The quantity of vapour which has passed into a liquid State from a 

 layer of air of 2 meters in height and 1 nr area, may be calculated, if we 

 remember that the température rises continually froni the ground upwards, in 

 such a manner that 0" will occur at a height of 1,5 m. and + 1" at a height 

 of 2 m, hence the average température of the layer of air will be 



At this temperatui'e the pressure of aq. vapour is 3,7°™ and the weight of 

 water per m^ = 4,o gr. 



The différence 6,8 — 4,o = 2,8 gr per m^ or per two m'^ = 5,6 gr = 



0,0056 kg. 



Per 10 hectares, this makes a quantity of water of 560 kg. 



In conséquence of the evaporation, mentioned above, from the bai'e ground 

 underneath, it may be necessary to increase this quantity to a certain degree, 

 which unfortunately is not fixed, but which we here présume to be 40 kg, 

 thus receiving a round sum of 



600 kg. 



