134 
Iron . 
been prior to the last fifteen years. By this mode the qua- 
lity of the air is less subject to alteration by a change of 
atmosphere. The principal objection to this manner of 
blowing, is the want of capacity in the receiving cylinder; 
which cannot be increased so much as to take away the 
considerable intervals which occur at different parts of the 
engine- stroke. This effect is sensibly seen by the speedy 
and irregular ascent and descent of the column of mercu- 
ry. In water blowing-machines, where the air is raised 
by three or four cylinders worked by means of a crank, 
where the air is received into an air- chest, and forced into 
the furnace by the continual action of the blast of each 
successive cylinder, the current of air is steady, and sup- 
ports the column of mercury with great uniformity. 
The use of the water- vault has of late years become very 
general among new erected works. Its properties are, a 
steady and very cold blast : the largeness of the receiving 
cisterns gives them a sufficient capacity to retain every 
pound of air raised by the furnace, and distribute it to the 
greatest advantage. This is not the case with the floating 
pistons, where a certain quantity of spare wind is thrown 
out at every return of the engine, lest the great piston and 
weight should be blown out of the cylinder altogether ; 
which, indeed, sometimes happens e The only objection 
which remains in force against the use of the water- vault, 
is the tendency which it has to take up a considerable por- 
tion of the water in solution, and introducing it into the 
furnace. A judicious arrangement of the conducting- 
pipes would in some measure obviate this, as well as the 
more dangerous tendency which water has to rise in a pipe 
speedily emptied of its air by the stopping of the engine ; 
a stream of water thus conveyed to the furnace, would be 
productive of the most awful consequences. 
The air afforded by the air- vault is much inferior to 
that obtained in the former methods. This immense ma* 
