144 
POPULAR SCIENCE REVIEW. 
tlie specimens exhibited are certainly of the highest order, 
whether as regards design or workmanship. The working 
parts are chiefly composed of wrought iron with a link-motion 
for working the valves. Paddle-wheel engines are not ex- 
hibited, excepting only in models, and a pair of oscillating 
engines, including drawings and models of Maudsley’s annular 
cylinder engine. Most of these have been in use for years, and 
are, therefore, deficient in novelty when compared with the 
more compact form of the screw-propeller class. Altogether 
the Exhibition in this department is replete with admirable 
specimens of marine constructions, and we have only to 
instance the COO nominal horse - power engines by Penn, 
and the 800 horse -power by Maudsley & Field for the iron- 
plated frigate Valiant, with others of less power, to be assured 
of the wonderful development of mechanical science in this age 
of progress. 
To marine constructions we have to add a great variety of 
vertical, horizontal, and angular engines, adapted to almost 
every possible purpose where power is required. Some of 
these engines have double, high, and low pressure cylinders, 
with and without condensers 3 others are single cylinders of 
peculiar construction, and exhibit several new arrangements 
accompanied with surface-condensation * and other contrivances 
for superheating steam t and preventing the escape of heat. 
All these are improvements on the past 3 but those which have 
been effected during the last seventy years (although valuable 
in themselves) are not such as affect the general principle 
arrived at by Watt, and subsequently perfected by the same 
comprehensive intellect that made the steam-engine what it 
now is, the strong arm of power and the hard-working agent 
of civilized existence. 
Water-Power . — Half a century has scarcely elapsed since 
water was the prime agent as a motive force. To that element, 
and to wind, we had recourse when power was required for the 
purposes of mining, agriculture, or manufacture 3 but the limi- 
tation of supply and requisite height of fall required for power 
were so great, and so uncertain, as to cause frequent stoppages 
of the works, and to spread them widely distant from each other 
over the face of the country. At the commencement of the 
present century, when the improved machinery of Arkwright 
and Crompton created a demand for power on a large scale, 
* Surface -condensation is a vessel where the steam is condensed by 
pumping cold water on to the exterior surfaces of tubes through which the 
steam passes. 
t Steam is superheated by passing the pipes containing the steam through 
flues or vessels containing air at a high temperature. 
