88 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



floating level, the diver being essentially unstable, and either rising or falling 

 with acceleration. 



To operate the apparatus the air is exhausted at t until the rider just strike? 

 the thermometer bulb. The top should, therefore, be flat. A very slight leak 

 is then made at t, and the air-pressure and temperature (at T) read off at the 

 moment when the rider and thermometer separate. With a light diver this is 

 determinable with remarkable exactness, particularly if the influx of air is 

 very slow. To determine the air-pressure, a mercury manometer and barome- 

 ter are here necessary. The former should be adjusted to 10 or 20 cm. 

 Pressures within o.i mm. of mercury and temperatures to a few hundreths 

 degree are required. 



As the exhaustion proceeds, the water-level at c rises, owing to the expansion 

 of air both at a and at c. The head h refers, therefore, to the instant of flota- 

 tion. Such a change of level is naturally undesirable, as is also the double 

 mercury apparatus, manometer and barometer, for air-pressure. 



To obviate this, the type of apparatus, figure in, was designed. The vessel 

 A A, swimmer ab, air-supply cylinder cd, thermometer T, etc., are practically 

 the same as before; but an auxiliary tubulure with a stop-cock g has been 

 added. Two lateral tubulures t and t r , however, are here available; one of 

 these /' communicates with the attached barometer D; the other, t, with 

 the mercury receivers B and C, connected by a flexible pipe, /. Between 

 the terminal mercury columns at B and D, the whole system, with the exception 

 of the air-spaces at a and c, is filled with water. If the mercury heads HI 

 and H 2 and water heads h' and h" be determined as shown in the diagram, 

 the air-pressure at the level a will be 



when B is the barometric height and TT the vapor-pressure of water-vapor. 



