ON THE TRANSMISSION OF POWER BY COMPEESSED AIR. 451 



roadways and footways as they would be in this country, and partly slung 

 from the roof of the sewer subways. They are supplied at intervals with 

 automatic float traps for carrying off the entrained water and the water 

 of saturation as they deposit. 



On entering a building on its way to a motor the air is first passed 

 through a meter (V.) exactly as gas would be. The quantity passing is 

 of course too great to allow anything like an ordinary gas meter to be 

 used ; indeed, only inferential meters seem to have been at all successfal. 

 The meter actually in use in Paris is a small double cylindrical box of 

 external appearance as in the figure. The air passes by a branch through 

 to the bottom of the inner box, up through it, down outside it between 

 the two boxes, and away through a branch at the bottom opposite the 

 inlet branch. The whole measuring-apparatus is a little foui*- or six-armed 

 fan, with aluminium or nickel vanes, placed near the bottom of the inner 

 casing, and communicating motion by a light vertical steel spindle to a 

 clockwork register, like that of a gasmeter, placed on the top. The 

 quantity recorded is simply the number of revolutions made by the fan, 

 or some proportional number, and this is turned into cubic metres by 

 multiplication by an arbitrary constant, determined by direct experiment. 

 As to the working of this meter I shall have something to say later on ; 

 it is the only type used by the Paris company, and serves in a very large 

 number of cases as a basis of payment. 



After passing the meter the air is carried through a reducing-valve 

 (VI.), by which the initial pressure in the motor is prevented from rising 

 above a certain limit, which in practice appears to vary between Bh and 5^ 

 atmospheres absolute, according to the size of the motor in proportion to 

 its work. 



Between the reducing-valve (VI.) and the motor (VIII.) there is placed 

 in all ordinary cases a small stove or heater (VII.) This heater is simply a 

 double cylindrical box of cast iron, having an air space between its outer 

 and its inner walls. The air under pressure traverses this space, and is 

 compelled, by suitably arranged baffle plates, to circulate through it in 

 such a fashion as to come into contact with its whole surface. A coke 

 fire is lit in the interior of the stove, and the products of combustion are 

 carried over the top of it, and made to pass downwards over its exterior 

 surface, inside a sheet-iron casting, on their way to the chimney-flue. 

 The heater for the motor on which I experimented (which indicated 10 to 

 12 horse-power) was about 21 in. in diameter and 2 ft. 9 in. high over all. 

 The use of the heater, and the extent to which that use is advantageous 

 and economical, are matters on which I shall touch later. 



The motors themselves (VIII.) used in Paris are mainly of two types. 

 Up to one hoi-se-power or thereabouts small rotary engines, of a form 

 patented by Mr. Popp, are used ; into the details of these it is not neces- 

 sary to enter here. They start very readily, are easily governed, are 

 provided with capital automatic lubricators worked by compressed air, 

 and run at a very high speed, and are altogether very convenient. They 

 use the air with little or no expansion, without previous heating, and 

 have, of course, no pretence to economy in use of air. 



The larger-sized motors, up to doable-cylinder engines 12 in. by 

 14 in., which is the largest size used, are simply ordinary Davey-Paxraan 

 steam-engines, employed for- air absolutely without any alteration or 

 modification. These engines have in most cases automal ic cut-ofl' gear con- 

 trolled by the governor, and can therefore easily work with tlie largest 



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