374 KINEMATICS OF MACHINERY.
Engineers in Birmingham, that "if (as here) the steam could be brought to bear on the crank direct, it would be a more simple and ready means (for utilising steam pressure) than at present in use " ? * He says this, too, at the very instant at which he has placed between piston and fly-wheel shaft a mechanism con- structively so difficult as the parallel cranks !
The particular machine referred to was said to work well ; but while this may be acknowledged, it must be added that it forms only one example of that tendency to aim rather at what we might call the bravado in machine construction than at the attainment of practically useful results which has too often proved fatal, in connection with rotary engines, to the judgments of sober and sensible men.
84.
Chamber-crank Trains from the Turning Double Slider-crank.
(Plate XXIII. Figs. 2 and 3.)
The turning double slider-crank ( 72) allows itself to be used very conveniently as a steam-engine or pump. It is indeed very frequently used in the form of the well known steam (" donkey ") pump sketched in Fig. 2, PL XXIII. The cross-block c is formed both above and below as the piston, and the fixed link d as chamber ; the steam- cylinder is double-acting, the pump very frequently single-acting only. Its general and special formulae are therefore (0JPi) d and (C;Pi)i respectively.
It will be noticed that the block b moves to and fro in its slot with the same sort of motion as the piston in its chamber, and in- deed with the same stroke. If therefore b were to be formed as a piston and c as its chamber we could use with these a pressure organ again, and so cross the dead points by chain-closure ( 46). It would be extraordinary if no inventive mind had yet hit upon this idea, and indeed we find that the American Eoot has based a design for a steam-engine upon it.f Fig. 3, PL XXIII. shows its general form. The two chambers d and c are prismatic, of
- The Engineer and Mechanist (J. S. Browne), vol. i., 1850, pp. 215 and 234.
Scientific American. New series, vol. x., 1864, p. 193.
