EXPLANATION OF THE PLATES. 347 



PLATE XVIII. 



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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 was invented by Professor 

 Parish, is given in Dr. Gregory's ' Mathematics for Practical Men.' 



