DoWLiNG — Steady and Turbulent Motion In Gases. 381 



A large gasometer was available, which was capable of holding about 

 twenty cubic feet. (Fig. 2.) The drum V was suspended by a thin wire 

 rope YR, passing over pulleys in a wooden scaffold, and carrying a counter- 

 weight W. A centimetre scale was attached to the wire Y and served to 

 record the vertical movements of the gas-container V. A wire Z was 

 stretched tightly across, as shown, and served as a mark behind which the 

 scale Y moved. Tlie rate of movement was calculated from the time taken 

 for a known number of divisions on Y to pass Z. Two methods were 

 available for altering this rate. One was by altering the weights W, tlie 

 other by varying the aperture of the tap T. 



Let B be the diameter of the drum V, and d that of the tube AB (fig. 1), 

 used in the experiment. Also, let t be the time in seconds for the drum 

 V to rise (or fall) / centimetres. The mean velocity of the air in the tube 

 AB will then be 



One difficulty only presented itself in working with this gasometer. Tlie 

 displacement of water by the walls of F altered appreciably the effective 

 driving-weight. This was, however, overcome quite easily ; for it only 

 occurred to a troublesome extent when working with small blasts. The 

 method then adopted was to "throttle" the blast by turning T, almost 

 " off " and adding a considerable excess to W. 



Another possibility may be referred to. The air in V will necessarily 

 be at a different pressure than the atmosphere. Usually, the air was being 

 drawn in ; so that the pressure in V was below the atmospheric pressure. We 

 will consider the most extreme case possible in practice, where the weights W 

 were somewhat short of 100 kilogrammes. The diameter of V was 84 cms., 

 so that its area was about 5500 sq. cms. Neglecting the weight of F, which 

 was considerable, the pressure-difference between tlie air in V and tlie 

 atmosphere was less than 20,000 dynes per square centimetre, or only two 

 per cent, less than the atmospheric. Thus in this extreme ease, never reallj' 

 attained in practice, an error of less thau two per cent, would result from 

 the calculation of V by the formula (3) above. 



The cock 2' was connected by wide rubber tubing to the tube C of the 

 ionisation chamber BC, used in the first experiment. The various tubes were 

 inserted at B in tight-fitting corks. Glass tubing, in ordinary stock-lengths, 

 selected as being straight and of as uniform diameter as possible, were used. 

 Their length varied between 120 and 160 cms. In most of these experiments 

 two lengths of tubing were employed, which were joined together by a tight- 

 fitting paper tube slipped inside, the joints being made air-tight by covering 



