EXPLANATION OF THE PLATES. 
347 
PLATE XVIII. 
To Boulton and Watt steam navigation is indebted for the effective method of 
working two engines jointly, giving, with other advantages, a more equable motion 
to the paddle wheel, and, in the event of accident to one of them, enabling the 
vessel to proceed with the other at about two-thirds of her greatest velocity. 
This plate is an isometrical projection 1 of a steam boat engine, in the manner 
they were first arranged by Messrs. Boulton and Watt; and nearly the same 
general principle of construction is followed by all the best manufacturers. Two 
small engines connected in this way were adapted to the ‘ Prince of Orange ’ and 
‘ Princess Charlotte,’ on the Clyde, by these gentlemen, in 1814. Previous to 
this, it had been the practice to employ only one engine, ranged by the side of the 
boiler, having a fly wheel on the paddle axis to assist the engine in passing its 
centres; and in such case the occurrence of any accident immediately put an end 
to the progress of the vessel by steam. 
The steam comes from the boiler by the pipe' in the front of the figure, and 
passes into the steam case and round the cylinder to the slide box, (see art. 146.); 
from whence it is let into the cylinder in a manner which will be more clearly 
understood by referring to the next plate: from the lower part of the cylinder a 
trunk proceeds to the condenser, which is below a square cistern ; beyond which 
a part of the air pump is seen, and to the left of it the hot water pump to supply 
the boiler. 
The motion of the parts commences at the cylinder: the piston rod is supposed 
to be descending, and by means of a cross bar (called a cross head) and two side 
rods, it depresses the ends of the side beams, these side beams moving on axes in 
the centre ; the other ends rise and force a cross bar upwards, to the middle of 
which the connecting rod is fixed, by which the crank of the paddle wheel shaft at 
the upper part of the figure is turned ; and also by the rising of the further end of 
the side beams, the cross head of the air pump and hot water pump is raised by 
two side rods. The motion of the piston rod is guided by a combination of rods 
called the parallel motion, (see art. 495.) and the slide is moved by an eccentric 
1 Some account of this simple and useful mode of drawing, which Avas invented by Professor 
Farish, is given in Dr. Gregory’s ‘ Mathematics for Practical Men.’