1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



19 



But we originally said, as we now repeat, that we do not mean to 

 divulge our own opinion- 

 One consummation there is .ievoutly to be wished, namely, that Dr. 

 Lardner will soon return to this country, to resume the wand which, 

 like Prosperos, none but its master can wield. We can see no other 

 antidote against that worst of Egyptian plagues, the swarms of vermin 

 with which the track of practical philosophy is now overrun. We are 

 death sick of the reign of minute philosophers. If the choice rested 

 with us, we would say, give us back our wolves again — restore the 

 dominion of barbarism — curse us with any thing e^ce\it the cant of 

 philosophical imposture — the disgusting egotism of idiotic mounte- 

 banks. Dr. Lardner's return to England would be the death warrant 

 of all such quacks, which is of itself a sufficient reason to inspire the 

 ardent desire that his return will not be much longer delayed. 



Z. 



► 



REMARKS OK THE TR.\CTIVE POWER OF PADDLE 

 WHEELS. 



In the theory of the nautical steam-engine there is no point that 

 deserves more attention than the means employed to obtain propelling 

 reaction from the water. The disadvantages which the various plans 

 at present in use labour under, have been very ably pointed out by 

 the iliil'erent writers in the appendix to the new edition of Tredgold's 

 treatise, and the mortifying result is that nearly half the effective 

 power of the engine is lost by the motion necessarily given to the 

 water by the paddles and their oblique action. Besides the lost 

 power from these causes, there are other disadvantages wliich the 

 nautical steam engine labours under. A change in the velocity of the 

 vessel, as well as a change in the immersion of the paddles, alters tlie 

 relation between the power of the engine and the propelling effect. 

 Under the most favourable circumstances, if the power of the steam 

 generated be reckoned 100, the efficient power acting on the paddle 

 wheel is about Gu, and the ellicient propelling power only from 35 to 

 •40; the power generated thus requires to bft nearly three times the 

 work done. It is evident that under given circumstances with a given 

 engine, a certain relation subsists between the extent of efi'ective pad- 

 dle surface and iliameter of the wheel, to obtain a maximum speed, 

 or in other words a maximum propelling duty. It is probable that the 

 experience of steam ship-builders enables them to lix upon very nearly 

 the best proportions for an average immersion, but is there any method 

 of proving that the best proportions have been exactly obtained. 

 There are not wanting very notorious instances to the contrary. It 

 may be s.iid that exactness in tliis is not of much importance, as the 

 propoitions that answer best in a calm will not do for rough weather, 

 and nee ni»<i. Suppose the best proportions have been obtained for 

 calm weather, and an average immersion the same propelling effect 

 •will not be maintained if the vessel, 

 Ist. H;is greater immersion. 

 2nd. Has less immersion. 



3rd. Is employed towing when not constructed for such work. 

 4tb. Has to make way against a fuul wind or head sea. 

 5th. Has a fair wind, and employs both steam and canvass. 

 This arises both from a greater absolute loss of power in appli- 

 cation, and likewise from the engine being obliged to work with re- 

 duced duty. Since, then, the engine is liable to so many adverse cir- 

 cumstances, it would be a great desideratum if the projiortiun that 

 exists between the effective duty on the paddle-wheels and their pro- 

 pelling duty coulil at all limes be known. The advantage or disad- 

 vantage of any change in the paddles would then be pointed out in 

 a way that could not be misunderstood. The first or effective duty of 

 the paddle-wheels m;iy V considered as proportional to the number 

 of revolutions per minute with the same steam pressure ; the pro- 

 pelling duty is measured by the number of revolutions conjoined with 

 the horizontal forward strain of the paddle-shaft on its bearings, or 

 the direct tractive force ex'-rted by tlie engine on the vessel. 



The difficulty is to measure this. Is it practicable to do so by 

 measuring the intea'<ity of the friction of two surfaces submitted to 

 the horizontal pressure ? The following has occurred to the writer as 

 one mode of doing this. How far it is practicable or likely to be 

 efficient, he must leave practical men to judge. 



vrm 



I 



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fTTNlml 



'JjiilJIillii. 



I'^l 





The figure is drawn to show simply the mode of action. The beam 

 A presses on B with a varying force. Two wheels cc, worked by a 

 pinion/; and wi:icli w, are embedded between the beams on one side, 

 and the central friction piece d on the other: the contiguous surfaces 

 being ground perfectly smooth. To the centre friction piece d, an 

 arm A- is attached the extremity of which e is connected with a fixture 

 /by the intervention of «, a spring weighing machine or other dyna- 

 mometer. By turning the winch the wheels cc are made to revolve 

 slowly, these, by the friction of their inner surfaces against d tend to 

 draw it along with them with a force which, according to the experi- 

 ments of Coulomb, Renuie, and others, is nearly proportional to the 

 pressure of A on B, and quite independent of the rate at which the 

 wheels c c are made to turn. This force by means of the arm i will 

 be shown by .t. To set off the scale upon s, it will be necessary to 

 apply known forces to A. This is the only adjustment required, and 

 may be made on the apparatus before being applied to the engine. 



Other arrangements of the apparatus will readily occur. In apply- 

 ing it to four bearings, one winch and s|)ring might be made to answer, 

 so as to give the measure of the tractive power at one obser^ation. 

 As an instrument for measuring pressure, this might perlwps be cn»- 

 ploycd for various other purposes. I fear, however, that it is too 

 simple to have escaped the notice of practical men, and that it lias 

 been tried and rejected. If not, the information it is intended to con- 

 vey has not been thought of such importance as to call attention to 

 any thing of the kind. But while so many conflicting opinions are 

 entertained on the subject of paddle wheels, it appears to be the only 

 thing that will properly bring out the true results of experiments, and 

 distinguish what is due to the build of the ship and what to the 

 paddle-wheels. Those paddle-wheels that evolve the greatest trac- 

 tive power with the full expenditure of steam .ire the best for the 

 given immersion, and this would be iu<licated by the spec I of the 

 vessel. But if it were required to find what immersion allowed the 

 engine to exert the maximum propelling duty, the speed of the vessel 

 would obviously not be a proper index, unless the relation bctweni 

 the immersion and tractive power were known. Again, if it were 

 <lesirable to compare the useful qualities of a paddle-wheel at the 

 same immersion but dilVorcnt velocities, a knowliulgc of the tractive 

 power is essential, ami would be sufficient. This would also Ije the 

 case in judging of the effects of a rough sea. If we might suppose 



D 2 



