THE FLOW OF GASES. 165 



in these experiments. For as the change comes on slowly, 

 it is at first too small to be appreciable in such short in- 

 tervals as 7*5 and 8 seconds. But an examination of Mr, 

 Wilde's table vi. shows that it lies between "5 and "53. 



This very remarkable fact^ to Avliich Mr. Wilde has re- 

 called attention, excited considerable interest fifteen or 

 twenty years ago. Graham does not appear to have 

 noticed it, although on reference to Graham^s experiments 

 it appears that these also show it in the most conclusive 

 manner (see table iv., Phil. Trans. 1846, vol. iv. pp. 573- 

 632; also Reprint, p. 106). These experiments also show 

 that the change comes on when the ratio of the pressures 

 is between '483 and "531. 



E,. D. Napier appears to have been the first to make 

 the discovery^. He found, by his own experiments on 

 steam, that the change came on when the ratio of pressures 

 fell to '5 (see Encyc. Brit. vol. xii. p. 481). Zeuner, 

 Fliegner, and Hirn have also investigated the subject. 



At the time when Graham wrote, a theory of gaseous 

 motion did not exist. But after the discovery of the 

 mechanical equivalent of heat and thermodynamics, a 

 theory became possible, and was given with apparent 

 mathematical completeness in 1856. This theory ap- 

 peared to agree well with experiments until the particular 

 fact under discussion was discovered. This fact, however, 

 directly controverts the theory. For on applying the 

 equations giving the rate of flow through an orifice to 

 such experiments as Mr. Wilde^s, it appears that there is 

 a marked disagreement between the calculated and expe- 

 rimental results. The calculated results are even more 



* The account of E. D. Napier's expei-iments is contained in letters in the 

 ' Engineer,' 1 867, vol. xxiii. January 4 and 25. They were made with steam 

 generated in the boiler of a small screw-steamer and discharged into an 

 iron bucket, the results being calculated from the heat imparted to a constant 

 volume of water in the bucket in which the steam was condensed. 



