Water-pipes and Syphons from disev gaged Air, 113 



essay on a very important subject in practical hydraulics, and the 

 thanks of the society are due to him for it, and particularly for 

 his contrivance for drawing air off from the syphon. In regard 

 to his views of the absorption of air by water, and its disengage- 

 ment in pipes, I cannot just altogether agree or follow him in 

 his notions; but the subject, it must be confessed, is not suffi- 

 ciently understood. The idea of a mechanical mixture of the 

 air with the water I conceive quite inadmissible; and, when we 

 consider that the former is 800 times lighter than the latter, it 

 is obvious they must be united by a very powerful attraction 

 overcoming t^e natural buoyancy of the air. When the pressure 

 of the external air is removed, the particles confined within the 

 water are seen to expand. Their buoyancy, therefore, increases, 

 and they overcome the force of attraction with which the buoyant 

 force was forming in equilibrio ; and this is the true cause of 

 their disengagement in the upper parts of the syphon. The 

 diminution of corpuscular attraction, on which Mr Hepburn 

 lays so much stress, I cannot think has much influence, as this, 

 in any case, is so extremely weak a force. In regard to the dis- 

 engagement of air in pipes, the curious circumstance here is, that 

 this takes place without the pressure of the atmosphere being 

 removed at all, and even when it is greatly increased by all the 

 pressure of the column of water to the height of the fountain 

 head. This is a very curious fact indeed, and not easily ex- 

 plained ; but the thanks of the Society are certainly due to Mr 

 Hepburn for bringing this subject under their notice, as it may 

 lead to useful discussion. 



Report hy Mr Clerk Maxwell, 



Many eminent engineers have been disappointed in their ex- 

 pectations of the quantity of water conveyed by pipes, owing 

 to various causes of obstruction to the regular flow, and which 

 make the result different from that deduced by calculation on 

 the most approved theory. The essay of Mr Hepburn is there- 

 fore of much value in elucidating the obstruction caused by air 

 in pipes, and in suggesting a method of removing it ; and in 

 particular, in pointing out a method of withdrawing air from 

 the syphon, and so rendering it a much more valuable instru- 

 ment than it has hitherto been. The same method of doing 



VOL. XX. NO. XXXIX. — JANUARY 1836. H 



