DAVIS. — CERTAIN THERMAL PROPERTIES OF STEAM. 273 



in the same run, it is throttled from exactly the same initial condition 

 A to the condition C ; then to D and so on. The well-known law of 

 throttling is that the total heat in the condition B, or C, or D, is equal 

 to that in the initial condition A. 



The point B represents superheated steam at the pressure ^> B ; the 

 point B' represents saturated steam at the same pressure ; and the 

 amount of superheat at B is the measured temperature there minus 

 the temperature at B', which can be taken from a steam table. Also, 

 by definition, the total heat at B equals that of saturated steam at the 

 same pressure (point B') plus the amount of heat required to superheat 

 it at constant pressure from B' to B. This is the integral of C p from 

 B' to B, or simply the mean C p from saturation multiplied by the 

 known superheat. If C p is known, this integral, or increment in the 

 total heat between B' and B, is easily evaluated. 



This integral is not only the difference between the total heat of 

 saturated steam at B' and that of superheated steam at B ; it is also 

 the difference between the total heat of saturated steam at B' and that 

 of saturated steam at A ; that is, between the two corresponding ordi- 

 nates of the curve that gives the total heat of saturated steam as a func- 

 tion of the temperature, the curve sought in this paper. To draw a 

 piece of this curve, one chooses arbitrarily some horizontal line such as 

 xy in Figure 4, and lays off below it, at the proper temperatures, the 

 distances bb', cc', del', etc., which represent on the desired H- scale the 

 integrals or total heat differences between B' and B, C and C, J)' and 

 D, etc. The curve ab'c'cV is an isolated piece of the true curve of total 

 heat against temperature. The relative height of its points, that is, its 

 shape, is accurately determined ; the absolute height above the usual 

 zero of total heats, namely, that of water at 0°C, is as yet wholly un- 

 known. The experiments of Grindley gave seven independent sample 

 pieces of this sort, one for each throttling curve, their temperature 

 ranges being known and greatly overlapping ; similarly Griessmann's 

 data gave eleven such sample pieces, and Peake's six. 



As was explained in the preceding paper on the Joule-Thomson 

 effect, Grindley's incoming steam (point A), and occasionally Peake's, 

 was not quite dry, so that its total heat was not determined by its 

 pressure and temperature. Whenever this seemed to be the case, the 

 points A and a of Figures 3 and 4 were left out of consideration alto- 

 gether. BCD would still be a curve of constant total heat, provided 

 only that the quality of the incoming steam at A remained constant 

 during a run, and b'e'd' would still be a useful piece of the desired 

 total heat curve. 



All sample pieces of any one observer were then plotted carefully on 

 vol. xlv. — 18 



