50 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[February, 



proporlion, tlip area of the sunn yorl nfjloat, necessary to produce an 

 equal resistance when moving llirougli tlie water at the rate of ■2'1 

 miles per hour, to be only OJ'T) sq. in. for rr'li ; 2'1()2.J ; ; 229 ; 95-5. 

 Hence we see plainly, that had the nclaiigulnr floats been of equal 

 area oiili/ with the trapezinni-shaped lluafs, and Iraixlkd at the aamt 

 rctocitii, tlie resistance would have lieen quite as great, if not greater; 

 for the area of the immersed trapezium floats is slated to be l(i7, and 

 the calculation shows that 9.5'o sq. in. would have been sulficient with 

 rectangular tloals. 



But "we have no occasion to stop hero; Mr. Rennie has tried the 

 merits of the two kinds of floats on a larger scale, (viz. with the 

 "Pink " steamer,) and we shall be able to show, that instead of being 

 superior, the trapezium-shaped floats prove themselves to be infinitely 

 inferior to (he rectangular, as the experiments are made on a larger 

 and fairer scale. 



Pursuing the same method of calculation, we find that in the wheel 

 with rectangular floats, the centre of pressure travelled at the rate of 

 7a4'S ft. per minute, or 8-5(3 miles per hour, whilst the boat only 

 travelled at the rate of 6-7 miles; then S-5(i — (>7 =: PSll, for the re- 

 cession of the rectangular floats with (iSirO sq. in. surface. The centre 

 of pressure of the trapezium-shaped floats travelled at the rate of 829 

 ft. per minute, or 9- 11 miles per hour, and the boat ti-Sl ; then 9-41 — 

 6'31 =3*07 for the recession of the trapezium floats per hour, having 

 a surface of ■f32-25 sq. in. The square of l'S(J = 3--J59t;, and the 

 square of 3'(l7=: 9-5219, then 9-5219 : 3-1590 : : G36 : 231 sq. in. 

 Here we again see, that had the rectangular floats had an area of only 

 231 sq. in., and trarel/ed at tlit. same nlotitij as tlie trapezium, the re- 

 sistance wovdd have been equal ! Whereas it appears by the experi- 

 ment, that the area of the immersed trapezium-floats, was 432 sq. in., or 

 (432 — 231 = ) 201 sq. in. greater than would have been necessary 

 with rectutiguhir floats. 



Thus far, then, we think we have proved all that we attempted, 

 and now let us ask, what are the other advantages besides a reduced 

 area, which are said to be derived for tlie use of the trapezium-shaped 

 floats? A reduction of two-thirds in the width of the paddle-wdieels 

 and boxes. Having clearly shown that no advantage whatever is 



gained as far as the area of the immersed floats is concerned, but rather 

 loss of power incurred ; what authority is there for asserting that 

 ])lacing the floats endwise is advantageous ? On the contrary, iTo not 

 the experiments prove this modification to be as good ;is disadvan- 

 tageous? for we see that when the obtuse luigle was down, a surface 

 of only 103 sq. in. w as suflicient to propel the boat at the rate of 2-8 

 miles per hour, with only Hi- J turns of the winch, and a 3 ft. Gi in. 

 wheel ; but when the acute angle was down, the velocity was only 

 2-9 miles, with 47-5 turns of the wincli per minnte, and a 3 f(. lUi in. 

 wheel. In fact, there is just as much authority for stating, and I make 

 bold to say, that (he results would be found equally as favourable, were 

 the common rectangular floats also placed endwise, as the trapezium 

 floats are when in that position. 



The disad\antages of the common paddle-wheel are universally 

 acknowledgetl, and it would therefore have been much fairer, had Mr. 

 Rennie compared the etfects of the trapezium floats with the cycloidal 

 or the vertically-acting paddle. 



Above all, it may not perhaps be rude to ask, who are the compe- 

 tent judges and witnesses who are said to "have seen the experiments 

 repeated again and again and testeei them?" I think I may venture 

 to say that Professor Barlow was not one of them, though he is said to 

 approve of the ])lan t/ieoret/catli/. 



In sending you my views on the subject, I trust I shall not be accused 

 of any motive unworthy of the subject, or of endeavouring to cast a 

 slur over the efforts of the talented inventor; but Mr. Rennie should 

 recollect, that assertions coming from so high a quarter, are much 

 more likely to mislead than when made by an obscure individual. If I 

 am wrong in my views and calculations, it will be easy to discover the 

 seat of error and thus elicit truth, and I shall then be the first to 

 acknowledge it ; but should this letter be the means of preventing 

 a needless expenditure of money, I trust that those interested will 

 consider that, instead of inflicting an injury, I have conferred on them 

 a benefit. 



Yemsleij, 



Jan. m, 1840. 



I am, Sir, 



Your obedient servant, 

 J. L. 



TRAVERSING SCREW-JACK. 



Fig.l 



TRAVERSING SCREW-JACK. 



FiGuuES I and 2 exhibit the screw modification. The screw- 

 jack a is bolted to the plank c ; at the other end of the plank is fixed 

 the rack g, in which the toe of (he strut/ advances as the screw Ij is 

 elevated ; the strut works in a joint in the follower k : the position of 

 the strut when (he screw is depressed is shown by the dotted lines. 

 The object of this strut is to relieve the screw of the violent cross 

 strain to which (lie apparatus is subject, when the engine or carriage 

 IS pulled over by the lever; which strain is entirely transferred to the 

 strut, and the screw has merely to carry (he load. 



The operation of traversing the jack is as follows ; by hooking the 



link i upon (he hook of the lever f, the toe of the lever being inserted 

 into a ratch of the rack A of the lower plank, when a man, bearing 

 down the end of (he lever, drags the apparatus and engine or carriage 

 towards him with great facilKy; the same lever is used to (urn the 

 screw, and to produce the traverse motion. By this apparatus an 

 engine of IG tons weight has been replaced upon the rails in five 

 minutes by the engineer and stoker alone ; thus those delays which 

 are the subject of so much annoyance and loss to railway proprietors 

 and the pulilic, need not happen in future ; the apparatus is exceed- 

 ingly jiortable and cheap, and no train ought to be allowed to go out 

 without its being sent along with it; it may be carried either upon 

 the lender, or upon some other f lace which may be selected for it. 



