64 On Compound Engines for Agricultural Purposes. 
considerably less steam ; and as the steam produced in a given 
boiler is proportionate to the coal which provides the heat for 
producing it, the coal used as well as the feed-water required 
are reduced in exactly the same proportion. 
Every agricultural engine requires coal and water to work it ; 
and the conveyance of these commodities to large portable and 
ploughing engines, in some localities, form material items of 
expense to the farmer ; and traction engines are frequently much 
delayed in getting them. Consequently from every point of 
view it is of great importance to use as little as possible of both 
commodities. 
Again, for portability it is important to keep an engine as 
light as possible. Every horse saved in moving a portable 
engine from place to place is a gain ; and ploughing and traction 
engines would have fewer enemies among road surveyors and 
County Boards, and would suffer far less from damages to bridges, 
&c., if they could be reduced in weight without reducing their 
power ; or when weight is necessary for tractive power, their 
owners would gain if they could travel farther without refilling 
their bunkers and tanks. At the same time such advantages 
would be doubtful gains if they were obtained by means of 
intricate mechanism impairing the simplicity of an engine. 
A smaller but by no means unimportant objection made to 
traction engines is the violent noise caused by exhausting steam, 
still at high pressure, into the atmosphere. Those who saw and 
heard the ploughing and traction engines at Wolverhampton 
when doing their utmost will well remember this, and engineers 
can appreciate the waste of steam and fuel which accompanies 
such noisy demonstrations of doing hard work. Such noise can 
be almost entirely removed when the steam is well expanded 
in successive cylinders. 
There is a popular impression that increasing the size of a^ 
engine cylinder increases its power. As a rule that is not the 
case, at least for continuous work. The gauge of the power of 
an engine is nearly always the steam-producing power of its 
boiler ; and in practice very few engines, either marine, land, or 
locomotive, exist, in which the cylinders are not capable of using 
vastly more steam than the boiler is capable of making. 
So soon as the public can be convinced of this, the tendency 
will probably be to reduce rather than to increase the cylinder 
dimensions in ordinary single cylinder engines ; and some 
makers, as Messrs, Fowler, are already advocating this prin- 
ciple in their agricultural engine practice. 
The introduction of the compound system will involve the 
replacement of the present single cylinder by one high-pressure 
cylinder much smaller, and one low-pressure cylinder some- 
