210 MR. W. F. G. SWANN ON THE SPECIFIC HEATS OF AIR AND 



could not be takeu until it had all drained down the gauge. It was found, however, 

 that when the oil had nearly settled down, two simultaneous readings on the two 

 limbs gave the same pressure difference as when it had become quite stationary. In 

 the experiments on carbon dioxide a device was adopted to shorten the time which 

 the oil took to drain. 



(12) The Preliminary Experiments. A long series of preliminary experiments was 

 performed for the purpose of testing the validity of the assumption made on p. 200 that 

 the electrical energy supplied per unit rise in temperature bears a linear relationship 

 to the rate of flow of the gas. It was first necessary to obtain an expression for the 

 rate of flow in terms of the pressure difference between the ends of the tubes and the 

 mean pressure in them, for on account of the inertia effects of the gas it was by no 

 means proportional to the product of these quantities. All the tubes except one were 

 blocked up ; the pressure was blown up in the bell, and the latter was allowed to 

 discharge gas through the fine tubes. The time taken for the bell to fall between 

 two fixed marks (the cross wires of two fixed telescopes) was accurately noted, and 

 the corresponding pressure readings were obtained. By performing a series of 

 experiments of this kind with different rates of flow it was found that the rate of flow 

 through the tube under examination was proportional to the quantity 



F = { V /(1 + 0'1043H)-1}P, 



where H is the pressure difference between the ends of the tubes, and P the mean 

 pressure in them. The rate of flow through all the tubes in parallel was assumed to 

 be proportional to this quantity, this assumption being sufficiently accurate for the 

 purpose in hand in the preliminary experiments. The variation of the rate of flow 

 with the temperature of the tubes amounted to about 0'5 per cent, per degree, but it 

 was measured and allowed for. 



The method of performing the calorimetric experiments was almost exactly the 

 same as that to be presently described for the final experiments. The following two 

 tables represent the results of a typical series of preliminary experiments. Assuming 

 a linear equation of the form CE/S0 = AF + h, where A and h are constants, and the 

 other quantities have the significance already assigned, the values of A and h which 

 best satisfied the observations were found. The extent of the agreement will be seen 

 from the tables. The heating effect of the leads was neglected in the preliminary 

 experiments, for since it represents a practically constant fraction of the energy 

 supplied in each flow, it does not appreciably destroy the linear relationship between 

 CE/S0 and F. 



The errors in the third columns of each table are estimated as percentages on the 

 energy supplied in the largest rate of flow. In the calculations several corrections 

 which remain sensibly constant throughout the series of experiments, and therefore do 

 not affect the linear relationship, have been omitted. The rise in temperature in the 

 above experiments was about 5C., but in order to test the method under conditions 



