The Kata-thermometer as a Measure of Ventilation. 199 



at one end to the central pillar of a Sherrington recording drum, and revolving 

 it in a horizontal circle ; (ii) other observations were made in the respiration 

 chamber, which is a closed room of 10'5x6"5x4 - 5 ft. capacity. The 

 experiments by the whirling arm method at Hampstead were very similar to 

 those carried out at Oxford. A screen of American cloth, 4 ft. high, was 

 erected on the floor of a large room, so as to form an enclosure 4 ft. square, 

 the top being open. The door and windows of the room were closed, so that 

 the air inside the enclosure was still. A transparent celluloid window was 

 inserted all round the screen at the height of the " kata " stem, so that 

 readings might be taken of the cooling, and the thermometer read. 



In the whirling arm method the actual velocity, v, of the " kata " is 

 given by 



2irm 



v = , 



t 



where r is the radial distance of the " kata " from the axis of revolution, n the 

 number of revolutions in the time t (estimated by stop-watch). L. V. King* 

 and others have shown, however, that this is not the true velocity relative to 

 the air, a certain " swirl " being set up by the revolution of the arm. This 

 was determined and allowed for in our observations. The procedure by which 

 this swirl was allowed for will be described later. 



The Dry "Kata" — Wind Velocity and Temperature. 



As shown in the previous papers, sources of radiant heat being absent, the 

 rate at which the " kata " cools in air depends almost entirely upon the wind 

 velocity and air temperature. It is shown that the heat loss per second per 

 square centimetre of bulb surface is obtained by dividing a " factor," deter- 

 mined for each instrument by the time of cooling, expressed in seconds, which 

 the alcohol meniscus takes to fall from 100° to 95° F., the unit of heat being 

 a millicalorie (1000 millicalories = 1 grin, calorie). This is known as the 

 f cooling power," H, and it was found that a connection between the variables 

 is given by an expression of the form 



K = (a + b x /v)6 



where v is the wind velocity and 9 the difference between the mean tempera- 

 ture of the range of cooling, viz., 36-5° C. (97"7° F.), and the air temperature 

 in degrees Centigrade, whilst a and b are numerical constants. 



To obtain the constants a and b, a number of observations of the cooling 

 power were taken, the temperature and wind velocity being accurately deter- 

 mined at the same time. The results are shown graphically in figs. 1 and 2. 



* L. V. King, ' Phil. Trans.,' A, vol. 214, p. 373 (1914). 



