314 
POPULAR SCIENCE REVIEW. 
scribed in these pages,"* and have been successfully worked with liquid fuel. 
In such a furnace the efficiency depends rather on the intensity of the heat 
produced than on its quantity, and it is quite possible that liquid fuel may 
be burnt with a much smaller proportion of air, and that a high and steady 
temperature may be the more easily and economically maintained than with 
coal. This result appears to have been attained at Woolwich, the furnace 
being worked at a less consumption of fuel, and the plates heated in a 
shorter time with oil than with coal. 
Steam Engine Terformance . — In the ordinary mode of comparing the duty 
of steam engines by determining tlie quantity of coal necessary to develope 
a given power, the efficiency of the boiler is not distinguished from the 
efficiency of the engine. Mr. B. W. Farey and Mr. Bryan Donkin have re- 
cently made some interesting experiments with an apparatus designed to 
measure directly the heat carried away in the condenser. Measuring at the 
same time the work developed by the engine, by means of indicator dia- 
grams, all the elements are ascertained, necessary for determining the 
efficiency of the engine independently of the boiler. The quantity of water 
passing through the condenser is measured by conducting it over a weir or 
notch. The temperatnre of the condensing water on entering the condenser 
and that of the mixture of injection water and condensed steam leaving the 
condenser, is ascertained by ordinary thermometers \ lastly, the work done 
in the cylinder is ascertained by indicator diagrams. Dividing the horse 
power developed in the cylinder, expressed in thermal units, by the quan- 
tity of heat imparted to the injection water in thermal units the quotient 
expresses nearly the efficiency of the steam. And since the efficiency of 
the steam in the same engine does not vary much for moderate variations 
of power, when the efficiency of the steam has once been ascertained the 
measurement of the volume and temperature of the injection water affords 
a new means of ascertaining the work done by the engine. Messrs. Farey 
and Donkin have therefore contrived photographic registering apparatus, by 
which the volume of flow and temperature of the injection water is con- 
tinuously recorded. The data so obtained being used either to determine 
the elliciency of the steam ; or, if the efficiency of the steam is known, 
to determine approximately the power of the engine. 
Mr. Herd on Iron- Clads. — Mr. E. J. Heed, the Chief Constructor of the 
Navy, has communicated to the Institute of Naval Architects a valuable 
exposition of his views on the advantages of short over long iron-clads, 
with an abstract of his paper previously presented to the Royal* Society. 
^Ir. Reed believes that it is unwise to make an iron-clad very long, large, 
costly, and unhandy, in order to effect a comparatively small saving in 
engine power. Hence he has introduced into the navy vessels in which the 
length is only five and a half times the breadth, instead of being six and a half 
times as in earlier armour-plated vessels. The reason why the disadvan- 
tages of excessive length are more apparent in iron-clad than in other vessels 
is that, in them, a great part of the weight to be carried is in the armour, 
and is dependent on the form of the vessel. Any addition to the length 
leads to a corresponding increase in the area of the surface to be armoured 
Vol. viii. p. 03. 
