EXPLANATION OP THE PLATES. 
365 
It is a remarkable fact, that from the first day of trial up to the present time, this 
boat has not varied her speed Wth of a mile per hour ; she has neither increased nor 
lost her speed. This is mostly to be attributed to the fact of the engines being in a 
most perfect state when set to work, but more particularly to the use of the patent 
slide valves on board of this vessel, and which after two years’ working have been 
found as perfect upon their faces as when first put together; a further proof of 
their superior working is evinced by the vacuum in the condensers of the engines 
having never varied ¿th of an inch, remaining constantly between 28^ and 28-|. This 
is ascertained by barometers attached to each engine, which are not affected by the 
atmosphere and makes the vacuum as perfect as possible, for supposing the waste 
water as it leaves the condenser to have a temperature of 110°, which is equal to 1^ 
inch of mercury, (see Professor Robinson’s experiments,) when added to 28j it 
gives 29f, the usual height of the marine barometer in fine weather. 
The safety valves are arranged upon the plan invented and used by Messrs. Boul 
ton and Watt a long time since, and now generally adopted by the engineers of 
London. They are so arranged that no one on board can possibly have access 
to them ; the engine man can at pleasure open them and let the steam escape, but he 
has no means by which he can keep them down, beyond the weight placed upon 
them by the engine maker, which weight is, as before stated, 3-| lbs. on the inch ; 
and it is a curious fact, that this boat has attained the great speed named, with this 
small pressure, while in a variety of instances vessels from different outports, working 
with high pressure steam, and with the safety valves loaded ad libitum by the en 
gineers and captains, have never been able to approach her in speed. This clearly 
proves, what the late Mr. Watt demonstrated long ago, that the most efficient, safe, 
and economical mode of working steam engines for marine purposes is at a pressure of 
from 2-| to 3-| lbs. on the inch. At the same time, for single acting pumping engines 
there is no doubt an advantage gained by the judicious use of high pressure steam, 
say of 30 lbs. on the inch, working expansively, with boilers properly constructed, 
but which boilers for many reasons are not at all fit for steam vessels ; in fact, almost 
all the melancholy accidents that have lately occurred to steam boats by the explo 
sion of their boilers, have been caused by the injudicious application of high pressure 
boilers to marine purposes. 
It is here worthy of remark, that the Americans, who claim to propel their vessels 
at the high speeds of 15, 16, and in some cases 18 miles per hour, (which, by the 
by, has been amply contradicted in this work by an able American writer, 
who states the greatest speed through still water attained by the best American 
steamers, he believes to be in one instance 14 miles per hour, but that the rest of 
the New York boats do not come up to this,) state that the principal cause of their