the TheTriw-dynamiccd Properties of Siq)crluoMcl Steam. 81 



at various pressures over a wide range of pressure ratio, the upper 

 pressure being kept constant. It was found that the maximum rate 

 of discharge did not occur until the ratio of pressures had fallen to 

 0*33, a value far below that given by the theory, and indicating a far 

 different law of flow through the orifice than the adiabatic. 



As a contrast to this, since the main element in the question 

 appeared to be the conductivity of the substance in which the orifice 

 is made, the later experiments were made with an orifice drilled in a 

 glass plate, the orifice being neither sharp lipped nor smoothly 

 rounded, the lip in the best circumstances presenting a rough chipped 

 edge. Now with such material for an orifice plate, it is evident that 

 any passage of heat between the glass and the steam will be very 

 small, and also that if adiabatic flow is not now obtained, then either 

 there must be a passage of heat between various portions of the glass 

 and the steam in contact with them, or heat must be conducted along 

 the stream of vapour itself, the latter being considered negligible from 

 considerations of gaseous conductivity. 



The experiments made with this orifice show a complete agreement 

 between the results of experiment and theory, and that the law of 

 flow through an orifice drilled in a plate of glass, no conditions being 

 attached as to the roundness or otherwise of the lip of the orifice, is 

 precisely the adiabatic law assumed in the theory. 



Part II. — On the Cooling of Saturated Steam hi/ Free Expansion. 



In Eegnault's experiments on the total heats of saturated steam 

 under various pressures, the steam was withdrawn upwards from a 

 boiler, allowing any entrained moisture in the steam to be separated 

 by gravity. Saturated steam obtained in any other manner would not 

 necessarily have the same total heat as that obtained by Regnault at 

 the same pressure, and it is therefore of great importance to note that 

 the dryness of the steam in Eegnault's experiments was obtained by 

 the simple method of draining suspended moisture from it. 



Hence, since the foundation of most of the researches on het thermal 

 properties of steam rests upon Eegnault's results, it would be well to 

 accept as a definition of dry saturated steam that condition of steam 

 which is obtained by draining from wet steam any entangled moisture. 



In making experiments on the thermal condition of superheated 

 steam obtained by wiredrawing saturated steam, a knowledge of the 

 total heat of eva,poration of the steam before wiredrawing is necessary, 

 and as Eegnault's tables of the total heats of saturated steam only 

 apply to steam obtained in the above manner, it must also be obtained 

 in the same manner for these tables to apply. 



The precise object of this paper is to describe a research on the 

 thermal properties of superheated steam, these properties being deduced 



