692 Beport of the Consulting Engineers on the Trials of 
Duty of the Boiler. 
Economy in the quantity of water needed by a portable 
engine, in relation to the power produced, is a question second 
in importance only to that of the economy of fuel. 
The supply of water to engines driving threshing machines 
in the field, or used for ploughing, commonly involves the 
expense and labour of cartage, and in the case of traction 
engines necessitates either the carriage of a heavy weight of 
water on the engine itself, the tank being made of large size, or 
involves the risk, should no brook or well be at hand, that 
the engine may be brought to a standstill on the road for want 
of water. Even when the well or brook is met with, there is 
needed the labour of pumping, or of otherwise raising the 
water into the tank of the engine. 
Considering a portable or a traction engine as a whole, the 
economy of water in relation to the power produced obviously 
depends solely on the efficiency of the engine ; while that of the 
fuel depends not only on this efficiency, but also on that of 
the boiler. The merits of a boiler may be entirely nullified by 
the defects of the engine which uses the steam : while the merits 
of an engine may be rendered unapparent by reason of the unsa- 
tisfactory nature of the boiler which supplies the steam consumed 
by that engine. Therefore, although the prizes are given for the 
boiler and engine as a whole, it is desirable, in order to obtain 
for mechanical science the full value to be derived from the 
" engine trials " of tlie Royal Agricultural Society, that the 
performances of the boilers, and of the engines, should be inves- 
tigated separately. 
A well-designed engine has its boiler fed from three sources : 
Istly, from the water obtained from some source of supply — 
which water corresponds to the quantity of steam puffed away 
from the chimney ; 2ndly, from the portion of the exhaust-steam 
which is condensed in heating the feed-water ; and, 3rdly, from 
the water produced by the condensation in the steam jackets 
which surround the cylinder. In the trials here recorded, the 
water obtained from without was measured directlys In those 
engines which used their waste steam to heat the feed-water, the 
quantity of steam condensed in so doing has been calculated from 
the observed rise in temperature of the known quantity of fresh 
water supplied, and the results may be looked upon as approxi- 
mately accurate. The quantity of steam condensed in the steam 
jackets cannot, however, be ascertained with a similar approach 
to certainty, because the jackets were in all cases connected 
directly with the boilers, so that no actual measurement could be 
made. It is certain, nevertheless, that the condensation in the 
