CHAP, in.] PUMPS. 63 



lighting the furnace is calculated from the time necessary to reach 

 the scene of fire, bearing in mind the time that it is required to obtain 

 steam of 100 Ib. pressure. When arrived at the fire the engine is 

 placed in a convenient position for working near the water, with the 

 fore carriage moved round at right angles to give greater steadiness ; 

 the necessary lengths of suction-pipe are then connected together 

 with the strainer at one end (entirely immersed in the water) and 

 the engine at the other. 



The importance of these applications for populous towns where 

 the violence and extent of fires require prompt succour and efficient 

 means of extinction requires no comment. 



III. PNEUMATIC MACHINES, OR GAS OR AIR-PUMPS. 



Pneumatic machines are really air or gas -pumps, with the 

 peculiarity that the fluid which they draw from a hermetically 

 closed space and force to the exterior gradually diminishes in density 

 without, however, bringing this density to zero, that is to say, without 

 producing a perfect vacuum. Scientific experiments require air- 

 pumps to be constructed with great exactness, in order that the 

 exhaustion obtained should approach as near as possible to a vacuum. 

 With the most perfect of these instruments the pressure of gas or air 

 which remains at the end of the experiment in the receiver may be 

 reduced to 01 millimetre. But it is not necessary to obain such a 

 perfect vacuum in industrial applications, and it is then more 

 advantageous to make use of an air-pump, invented and constructed 

 by M. Deleuil, an ingenious maker of delicate instruments of 

 precision. Plate III. gives a view of this machine, and in Fig. 43 

 the piston and barrel are drawn on a larger scale. It differs from 

 ordinary air-pumps by the introduction of an interesting and original 

 principle. The piston (instead of being lubricated with oil in order 

 that a perfect contact between its surface and that of the pump- 

 barrel may prevent all escape of air) in reality does not touch the 

 pump-barrel at all ; it is moreover furrowed with parallel and 

 equi-distant grooves. The very small interval (O mm> 02) which the 

 constructor thus leaves between the two surfaces is filled with a thin 



