1845.1 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



ia height weia required witli arcades of lu feet by 19 feel. I'li- 

 doubtedly, the present vTindows — und t.cken by llieniselvcs they are 

 of pleasing and unhackneyed character — miglit liavc been set wilhin 

 the arcades without otiier filling up ; whereas, greater degree uf rich- 

 ness, accompanied by great jilav of plan and diversity of surfaces, has 

 been obtained by converting the an-ades into large niches or coved 

 recesses, cut by a plane or vertical pta/vnd, occupied by the window 

 and its immediate dressings. We perceive the same idea, though 

 very differently treated in detail, adopted by Wren in some of the 

 windows of St. Paul's, which may satisfy the scruples of those who 

 can admire nothing unsupported by some sort of authority or prece- 

 dent, but does not at all detract from the merit of what has hen' hern 

 done. If there be anv part which we wish had been treated somewhat 

 differently, it is tlie window-dressing or chambrault itself, for although 

 exceeding!)' well composed, taken by itself, it appears to us that it 

 would liere have been better without the pediment, because the pedi- 

 ment cuts into and obstructs the small arrli or lunette of the plafond 

 somewhat disagreeably, and occasions some confusion of lines. The 

 window might perhaps have easdy been raisi-d a little, so as to bring 

 its cornice on a level with the impost mouldings of the arcade, ami the 

 hinettes might have been filled in with a series of medallion portraits 

 of English sovereigns,— yet they would perhaps have hardly been seen 

 sufficiently distinct at tliat height. Those who remember the design 

 of the upper part of the quadrangle of the old Exchange, or can turn 

 to any prints of it, will not admire tlie present design the less for com- 

 paring it with that truly hideous piece of architecture — so excruciat- 

 ingly detestable that it in impossible to believe Wren was at all con- 

 cerned with it, and to attribute it to him is only to disgrace iiis repu- 

 tation. 



*»* In tlie general description of the building, given in a former 

 number of the Journal, we enumerated the tradesmen employed, but 

 omitted the naities of Messrs. Bunnett and Corpe, who furnished the 

 patent revolving iron shutters to the whole of the shops. 



MECHANISM FOR MULTIPLYING MOTION. 



This subject has recently received much additional importance 

 from the invention of the Archimedean screw for propelling steam 

 vessels. It has been found by practical experiments, and has also boen 

 shewn independently by mathematical investigation, that the utility 

 of the propeller depends in a great measure on tlie rapidity with 

 which it revolves. Now the ordinary method of converting the alter- 

 nate motion of the piston rod of a steam engine into a circular is by 

 a crank ; the number of revolutions of which equals the number of 

 complete strokes of the piston-rod, as every time the piston-iod 

 ascends from its lowest to its liighest position and drscends again, it 

 produces one complete revolution of the crank. The revolutions thus 

 produced are, however, found to he too slow for efiiciently working 

 the Archimedean screw without the intervention of cog-wheels or 

 straps, which are therefore employed to multiply the number of revo- 

 lutions of the crank. The employment of cog-wheels is, however, 

 liable to many serious objections ; the friction of the cogs against each 

 other causes them to wear away, and therefore work untruly ; tliey are 

 also liable to constant breakage ; and lastly, the noise which they 

 cause in working is a serious impediment to their general adoption. 

 The last objection has been in some measure met by the use of wheels 

 with wooden cogs, to which however the two other objections apply 

 with increased force. The following method of multiplying motion 

 by a system of cranks only, is by no means offered as a complete solu- 

 tion of the important mechanical problem, but as nothing of the kind 

 has been hitherto suggested, it may possibly contribute something 

 towards tlie object in view. 



Each complete stroke of the piston-rod, it was said abovi', pro- 

 duces one revolution of the crank — the following is a method by which 

 it may be made to produce two revolutions. 



Fig. 1. 



Ill tig. I, A.J represents the end of the piston-rod, (the rod itself is 

 omitted fur the sake of distinctness,) which end is capable of moving 

 in a straight line from A to A,. It is here supposed that the end is 

 contined to this coursi' by a groove or guide, as indirect action engines. 

 A., is the middle point between A and A^, opposite to this point is 

 aiiother groove B, B^, at right angles to A, A^, and about half its 

 length. In this groove works one end, li^,, of a rod, the other end 

 being connected with A^,; now it will be seen thatwhi'u A^ passes 

 from A to A ,, li^ passes from B to B^ and back again, and that there- 

 fiire one vibration of A^ corresponds to tmo of B^,. For when A., is 

 at A tlie rod is in the position of the dotte<i line, and as A^ advances 

 from A to Aj, B^ ascends from its lowest to highest position, but it 

 begins to descend again when A^ has passed the middle point A,, and 

 finally is at its lowest position again when A^, is at A^. The same 

 happens on the return of A^; so every complete motion of A„ pro- 

 duces two of B^, and therefore a crank attached to B^ would revolve 

 tivice as often as one attached to A„. 



It is obvious that the means, by which the alternate motion of A^ pro- 

 duces a double movement of Bj, may be used to m ike B^ produce 

 a double movement in a third crank, and this again in a fourth, &c., 

 so that we have here a method of multiplying the number of 

 strokes of the rod (and therefore revolutions of the crank), bv 

 2, i, 8, 1''), &c., that is, in mathematical language, bv any power 

 of 2. But in practice, after two or three multiplications the ma- 

 chinery would become too complicated to be efficient, and there is 

 also another defect in the mechanism, owing to the extent of mo- 

 tion of B* not being more than half as much as A..; indeed it will 

 be exactly half as much (as may be seen by the slightest knowledge 

 of geometry), when B, the end of the second groove, is brought close 

 up to A,, the middle part of the first groove. Novf if many grooves 

 were used, this diminution of their successive sizes would be so often 

 repeated that the last rod would not have sutBcient extent of motion 

 to turn a crank of any useful size. 



This evil may, however, be overcome, or at least palliated. It will 

 be observed in fig. 1, that it is not absolutely n-cessary that the groove 

 A, A.„ iihonld be straight ; the action would be precisely the same if 

 the groove were an arc of a cir 'le, or, which is tlie same thing, if the 

 point Aj, instead of being the end of a piston rod, were the end of a 

 beam, with one extremity turning on a pivot and the other moving 

 l)ackwards and forwards from A to A_, in an arc, the motion of the 

 second rod would be tlie same. This is exemplified in the next 

 figure. 



Fig. 2. 



In fig. 2, a beam C D, moving about a pivot at C, one of its endi, is 

 connected at B with a rod, of which the end A moves, as before in 

 fig. 1, in a groove. The extent of motion of D, from E to F, is nearly 

 as great as that of A, and another connecting rod might be attached to 

 D to again multiply the motion. By such a contrivance as this the 

 multiplication may be continued several times without an inconvenient 

 diminution of the extent of motion. 



This method of multiplying reciprocating motion may be applied 

 directly to rotary motion, without the intervention of additional 

 machinery. Figs. 3 and I exhibit a scheme for this purpose ; A is, 

 as in jfig. ], the end of the piston rod, moving in a groove between 

 A^> and A., (it is immaterial whether the groove be straight or A be 

 moved in arc by the end of an engine beam); C, the centre about 

 which the crank revolves, is opposite A^, the middle point of the 

 motion of A. Figs. 3 and -1 shew this crank in its different positions, 

 thi' same letters referring to the same tilings in both figures. A, B, 

 is the rod connecting the crauk with A. We will trar- the progress 

 *f' he crank by referring alternately to the two figures. 



