BARRENS, NAPIER, YULE. 353
If it be desired to show that the same links a and d are common to both chains, that is, that only one of each of these links exists in the machine, we must add a d at the end of the formula outside the braces.
Bahrens used only one block, so that his chain was only single, and he employed a weight to effect the closure of c. Napier, on the other hand, used a perfectly correct chain-closure for that purpose. At first he also used a single chain only, omitting, like Bahrens, the lower block and the corresponding ports, he con- nected, however, with the block two pins conaxial with 3, one being placed on each side of the chamber d (externally), and he connected each of these by a coupler equal in length to b with eccentrics having their sheaves conaxial with the cylinder 2. In other words, he employed two auxiliary and in every respect equal and similarly acting chains of the form (C" 3 'P- L ) d , which not only had throughout the same link lengths as the chamber-crank chain, but had also the fixed link d in common with it. It is evident without further expla- nation that such chain-closure must ensure the constrained motion both of the block c and of the crescent-shaped coupler I.
Bompard first employed the double chain shown in our figure, he used a special chain of several links (which we need not here describe) to effect the closure of the block c, and as it does this only approximately, his engine forms a less perfect solution of the problem than Napier's.
PL XV., 4. Kotary steam-engine proposed and constructed both by Yule* (Glasgow, 1836) and by Hallf (before 1869). It is a reduced turning slider-crank with the coupler omitted and with force-closure used for the block c, and will therefore be written
The link a is again the driving link and the piston, while d forms the chamber. Yule arranged his ports as in our figure ; Hall made the exhaust as we have shown it, but caused the steam to enter through an opening in the block itself, for which reason he carried up the chamber above the block (Of. Fig. 4) and connected the
- Battaille et Jullien, Machines d Vapeur (1847), vol. i., p. 449 ; Berliner
Verhandlungen (1838), p. 233 ; Bourne, St. Eng., p. 392.
t Propag. Industriclle, iv. (1869), p. 340; Genie Industriel, vol. xxxv. (186S), p. *2.
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