ON PNEUMATIC MACHINES. 271 



[the rod EF, which moves the valve, in figs. 1 and 2, being united to it at O. 

 I* is called an eccentric, and consists of a hollow circle working on a solid 

 one, the centre of motion of which is not the centre of the circle. As the 

 centres of both circles always coincide, and that of the solid circle revolves 

 about the centre of motion, the rod CM will be moved forwards and back- 

 wards, and consequently will move upwards and downwards, and effect 

 the different communications and interruptions by the aid of the D valve. 



The application of the steam engine to navigation, which took place about 

 the time of the appearance of Dr. Young's work, was a result so obvious and 

 necessary, that it required the development of no new principles, and but 

 little refinement in the application of those already recognised, to bring it 

 about. The size and weight of the machinery no doubt offered a considerable 

 obstacle at first, inasmuch as the force obtained might bear too small a pro- 

 portion to the mass to be moved and the resistance to be overcome, to render 

 it economical. And this in fact appears to have been the case with the 

 earliest application of steam power to navigation, that of Mr. Symington. 

 But the perseverance of Fulton, Henry Bell, and others, obviated all these 

 difficulties, whilst successive improvements both in the arrangements and 

 the construction of the different parts of the machinery, have rendered 

 the expenditure of fuel very much less than it was a quarter of a cen- 

 tury ago. 



For a considerable time steam vessels plied only on rivers, not daring to 

 venture into the open sea, and nautical men, for the most part, entertained 

 the opinion that they were unfitted to brave it. George Dodd, an enter- 

 prising but unfortunate man, decided this point. He came down to Glas- 

 gow and fitted up a little vessel of 75 tons burthen, with a steam engine of 

 14 horse power, in which he started with a crew of five seamen, two engine 

 men, and a boy, for London. Although the voyage was stormy, it was 

 safely performed in 122 hours (exclusive of stoppages). Dodd was emi- 

 nent as engineer ; he projected Waterloo Bridge and the Thames Tunnel,* 

 purposing to carry it across from Gravesend to Tilbury, at the estimated 

 cost of under 16,000 ! t Yet, with talent, energy, and courage, he almost 

 literally died in the streets a beggar. His active mind led him into dis- 

 astrous schemes failure impoverished him and drove him to intempe- 

 rance, which ended in destitution and premature death. 



The adaptation of the steam engine to the propelling of vessels is now 

 so universally known, that a very brief description of the mode of effecting 

 it will suffice. Across the deck of the vessel is carried a shaft, to the ex- 

 tremities of which paddle wheels are fixed, the action of which every one 

 is familiar with. On this shaft two cranks are constructed at right angles 

 to each other, on which the connecting rods of the two engines respectively 

 work. By this contrivance a tolerable uniformity of action is produced, 

 without the aid of a fly wheel, the one engine being in its position of 

 greatest effect when the other is in the contrary position. The principal 

 feature in the construction of marine engines, as compared with land ones, 

 consists in the reversal of the beam, to prevent the inconvenience of its] 



* Stuart's Anecdotes, p. 534. Probably his father was the projector, 

 t Nicholson's Journal, ii. 239, 473. 



