Nomenclature and Notation in Calorimetry 419 



thickness of the layer is 16 fathoms; therefore the loss of 

 heat has been 16 x 5-8 = 92-8 P F., or 92-8 fathom -degrees- 

 Fahrenheit. The total depth of the lake at the spot was 

 35 fathoms, therefore the layer of water below the depth 

 of common temperature was 19 fathoms thick. The mean 

 temperature of this layer was 47-! F. on September 5, and 

 48'9 F. on October 15, showing a rise of i-8 F. in the 

 interval. This corresponds to a gain of heat represented 

 by 19 x 1-8 = 347 fF. Assuming that the heat gained by 

 the lower layer has been entirely at the expense of the upper 

 one, we see that the loss of heat of the upper layer, during 

 the interval, has been to the extent of 37-4 per cent, to the 

 deeper water, and 62*6 per cent, to the air. The upper layer 

 of water has thus been passing heat at the average rate of 

 [485 f F. into the air, and into the deeper water at the rate 

 of 0-85 f F. per day. 



It is worthy of remark that the fathom-degree-Fahrenheit 

 and the metre-degree-Celsius are interchangeable in heat cal- 

 culations, because the fathom is r8 metre and the Celsius 

 degree is i'8 F. 



This is a great convenience, and its usefulness will be 

 apparent by applying it to the above example. 



We have seen that, during the interval of forty days, the 

 average transmission of heat from the upper layer of water 

 has been at the daily rate of 1-485 f F. to the air and of 

 0-85 f F. to the deeper water. Writing m C. for f F., and 

 considering a horizontal area of one square centimetre, we 

 find at once that the average daily supply of heat from the 

 water to the air has been at the rate of 148*5 gC., and to the 

 deeper water at the rate of 85 g C. (gramme-degree-Celsius) 

 per square centimetre of superficial area. 



It is unnecessary to provide for special cases where specially 

 suitable units will be chosen as a matter of course ; but for 

 ordinary work of constantly recurring type it is important to 

 have a system of nomenclature and of notation, each of which 

 will tell its own story. 



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