190 HOT-AVATER BOILEUS AND riFES. 



5. Accumulation of air in pipes. — It is necessary to make 

 provision for the escape of air in the pipes, which sometimes so 

 accumulates as to prevent circulation. This is more especially 

 the case when the apparatus is complicated, and has many turn- 

 ings and vertical bends in the pipes. It generally collects at the 

 upper bends of the pipe, but this will depend very much upon 

 the mode of supplying the apparatus with water. It frequently 

 requires the greatest care and the closest attention to discover 

 where the air is likely to lodge, as the most trifling alteration 

 in the position of the pipes will entirely alter the arrangements 

 in regard to the air-vents. Want of attention to this has been 

 the cause of many failures, and the discovery of the places 

 where the air accumulates is sometimes a matter of difficulty. 

 For although it be true, in a general sense, that air will rise to 

 the highest part of the apparatus, it will frequently be prevented 

 from getting to the highest part by alterations in the level of 

 the pipes, and by other causes. 



As water, while boiling, always evolves air, it is not sufficient 

 merely to discharge the air from the pipes on first filling them, 

 because it always accumulates ; and, in many instances, it is 

 desirable to have the air-vent self-acting, either by using a valve, 

 or small open pipe ; but we have generally found a cock most 

 convenient. 



The size of the vent is not material, as a very small opening 

 will be sufficient to allow the escape of air'. The rapidity of 

 motion in fluids is inversely proportional to their specific gravi- 

 ties, as water is 827 times more dense than air ; an aperture 

 which is sufficiently large to empty a pipe in 14 minutes, if it 

 contamed water, would empty it, if it contained air, in one 

 second. Air being so much lighter than water, it is of course 

 necessary that the vents provided for its escape should be placed 

 at the highest parts of the apparatus, for there it will alvvaj^s 

 lodge when no impediment occurs to prevent it ; but it will 

 sometimes be found necessary to have several in difTerent parts 

 of the apparatus. 



Though it is perfectly easy to provide for the discharge of the 

 air from the pipes, — as far as the mere mechanical operation is 

 concerned, — it requires much consideration and careful study to 



