337

Chap. 8.] Moden of Working Fire-Engines,

the following, exhibiting the relation between the height of a jet and theair’s compressure, has long been published. It is, however, of little useto practical men. We doubt if a column of water of the size of thosethrown by ordinury engines could be raised by any means, two hundredfeet above the orifice of the pipe whence it issued: the resistance of theatmosphere would disperse it before it could reach that elevation.

Volume of air contained in the air Ratio of the air’s Height to which it is said the

chamber compressed to elasticity. water will spout.

1

2

- - - 2 - -

33 feet

1

3

- - - 3 - -

66

U

1

4

- - - 4 - -

99

u

1

5

- - - 5 - -

132

(<

1

6

- - - 6 - -

165

ii

1

7

- - - 7 - -

198

u

I

8

- - - 8 - -

- 231

u

\

9

- - - 9 - -

- 264

k(

1

10

- - 10 - -

- 297

u

Grreat as are the advantages derived from air chambers, some attentionto them is required in Order to secure at all times the benefit they aredesigned to impart. When neglected (and we believe few parts of anengine exercise the attention of firemen less) they often become actuallyinjurious, for when no advantage is derived from the elasticity of the con-fined air, the water is impeded in its progress by passing through them.Upon the trial of engines it sometimes occurs that the water is thrownhigher at their first working than after they have been a few minutesin use, and this notwithstanding all the efforts of the firemen to make thejet reach the first elevation. This result has sometimes been attributed tofatigue in the men—to obstacles in the pipes—to grit or sand under thevalves, &c. whereas in fact it was often due to the air vessel alone ; i. e.to the escape of air from it. This escape may be occasioned by mi-nute leaks in the chamber, but when no such imperfections exist the airfrequently makes its exit, and its place becomes occupied by the liquid.Whenever air is subjected to great pressures in contact with water, it isquickly absorbed by the latter, and in this way it is that it often disappearsfrom the air chambers of fire-engines, and also from those of pressure-en-gines, Heron’s fountain, water rams, &c. When a long suction hose isattached to an engine and the latter worked at a moderate velocity, asufficient supply of air to replace that taken up by the water, commonlyenters, unknown to the firemen, through the seams and joints ; but whenone engine is fed by another pouring water into its cistern, there is littlechance for the requisite supply of air, unless a minute opening were leftin the cap that screws over the orifice of the suction pipe, at one end ofthe engine.

The suction cocks of some engines dimimsh their üseful effect in con-sequence of the holes through the plugs being smaller than other pas-sages for the water.

The great desideratum in modern fire-engines is an improved mode ofworking them. At page 72 we remarked that experimental researcheshave shown the useful effect of a man working a pump, in the ordinaryway with a lever, to be fifty per Cent less than when he turns a crank;and that when his strength is applied as in the act of rowing, the effect isnearly one hundred and fifty per cent more than in moving a pump lever.This is sufficient to induce efforts to supersede the present mode of work-ing the pumps of fire-engines, and particularly so, as the labor is so se-

43