20 



THE CIVIL ENGINEPm AND ARCHITECT'S JOURNAL. 



[Jan. 



steamers generally to have such traction gauges, and a careful register 

 of its indications kept during all weather, and at every variety of im- 

 mersion, each voyage would have all the character of exact experi- 

 ments, and the results would bear upon the build of the ship as well 

 as the efliciency of the paddles in the given circumstances. The 

 effects of fair or foul weather would also be made manifest. If the 

 best kept steam logs are examined with the view of obtaining infor- 

 mation in regard to the qualities of the paddles, it will be found im- 

 possible to make any deductions ; there is an absolute darkness merely 

 from there being no way of knovv'ing the tractive power exerted by 

 the engine. 



The theory of paddle-wheels has received great attention from 

 several of the writers in the new edition of Tredgold, but it may be 

 questioned if the practical deductions are equivalent to the labour 

 bestowed upon the calculations. Indeed, the circumstances wdiich 

 affect them are too various to be embraced by mathematical formulae. 

 Besides, the whole depends on a law of resistance which is hvpo- 

 thetical. The masterly disquisition by Mr. Woolhouse seems to ex- 

 haust the subject ; but the formuhc given by the same gentleman in 

 the foot note at p. 301 will be found the most useful for (be purpose 

 of instruction. These, if a little modified and amalgamated with the 

 concluding formulae at p. 194, would be of great service to the young 

 naval officer. With a little knowledge of algebraic notation, so as to 

 work out the numerical values of each expression, taking numerous 

 examples, he would obtain a facile acquaintance with the peculiar and 

 unique relations that affect the working of the nautical steam-engine. 

 Thus he would become aware of the difference in the proportions that 

 ought to distinguish the paddle-wheels of a towing vessel, the disad- 

 vantages that a common steamer labours under when employed in 

 toviing, or a towing vessel when steaming by itself, the evils of too 

 deep or too light immersion, &c. A small publication adapted to such 

 views appears to be a desideratum at present, and would be calculated 

 to benefit a numerous class who ought to have a thorough knowledge 

 of the subject. The method of working out numerical examples from 

 the formulae w'ould be the means of communicating such knowledge 

 without the necessity of mathematical attainments. 



It is sin ular that no notice is taken of the theory of paddle-wheels 

 in the treatises on the nautical steam-engine by Lieut. Otway and 

 Robinson, although in point of importance it deserves nearly as much 

 space as the engine itself. Indeed, it is a subject that deserves the 

 attention of the naval officer more than any other, as a difference in 

 the weather, immersion, &c. so notably affect the amount of work per- 

 formed by the engine. To be fully acquainted with the power he has 

 at command, he must know,Trom studying the theory of paddle-wheels 

 and their connexion with the velocity of the ship and of the engine' 

 bow much that power is liable to be diminished or increased by cir- 

 cumstances, that are wholly independent of the condition of the engine 

 itself. A single example will show the importance of this ; take that 

 of an 800 tons war steamer which makes 10 knots in a calm, the engines 

 working at full power. The velocity per second is 17 feet; the cir- 

 cumferential velocity of tlie centre of (he paddle boards is 23 ft. leav- 

 ing G feet for the back velocity of the paddle-boards. Now with a 

 head wind and sea sufficient to stop the ship way so that it is just able 

 to maintain its position without drifting to leeward. The greatest 

 power that the engine is capable of exerting, would be reduced to 4; 

 or i, what it was in a calm. Thus J of the steam raised in the boiler 

 becomes useless. The engine becomes paralysed from the connection 

 of the parts being adapted to only one velocity and immersion. If it 

 were possible to construct it otherwise, so that full duty could be 

 maintained in all circumstances, the steamer could make nearly ei" ht 

 knots against such a gale. 



To look forward and answer the question— What is required to 

 render the application of the steam engine to navigation perfect? 

 Two great sources of disadvantage present (hemselves: 



1st. The motion given to the water to gain propelling reaction. 

 2nd. The motion of the piston is not uniform under varying circum- 

 stances of immersion and speed of (he vessel, neither is the momen- 

 tum of propulsion communicated by the engine equal [in equal times. 



With regard to the first it may be remarked, that if the reaction is 

 supposed to take place merely from projectile force given to the 

 water, the quantity put in motion to generate the same propelling 

 force in equal times will be directly as the area of the paddle boards. 

 But the loss from the back action of the paddles will be inversely as 

 the same. If the practical advantage of having small paddle-boards 

 is merely to present less surface to the shocks of a boisterous sea, 

 their surface may be reduced in proportion to the square of the aug- 

 mented velocity. Thus, taking the previous example, the area of the 

 paddle boards that require l> feet back velocity, and communicate a 

 propelling duty of 17, may be reduced one half without reducing the 

 duty more than 22 or to HA, and the velocity of the ship from 10 to 

 9i. On the other hand, by doubling the area the propelling duty 

 would only be augmented to 1S|, and the velocity to lOi. 



The disadvantage of a considerable loss from receding in the water 

 will always exist so long as the propelling reaction is derived from it 

 in a continuous manner. 



If the same extent of surface is made to move through the water 

 with double the velocity, it will gain the same propelling reaction 

 from the water in one fourth the time, and to do so will have to move 

 over one half the space ; so that if it were possible to apply the power 

 of the engine in this manner, the propehing duty w"ould rise from 17 

 to 20. But (0 effect this, it would be necessary to arrange the ma- 

 chinery so as to expend on the water, in a quarter of a second, the 

 power generated by the engine in one second ; and on the other hand 

 to distribute the force of reaction derived from the water in a quarter 

 of a second, so as to serve as a propelling power during one second. 

 The action of the oars in rowing is a familiar instance of this ; the 

 consequence is a considerable fluctuation in the velocity which on a 

 large scale would be very disagreeable if not otherwise objectionable. 

 To obviate the second source of disadvantage it would be necessary 

 to arrange the machinery so as to vary the absolute velocity of the 

 paddle floats, according to circumstances, while retaining the same 

 back velocity at all times. To effect this it would be necessary to 

 arrange the machinery so as to vary the absolute velocity of the 

 paddle floats, while, retaining the same back velocity, to obtain this it 

 would be necessary to disconnect the wheel from the engine, and by 

 a shifting leverage to bring the lowermost paddles only under its full 

 action, while the others are allowed to revolve passively by catching 

 the water. It is cjuite possible to make an arrangement to effect all 

 here that the theory demands, but how far such would be concomitant 

 with practical efficiency, is a much more difficult question. 



It may be remaiked that the screw propellers are as much infiuenced 

 by these disadvantages as the common paddles, but they are not so 

 much affected by oblique action or by waves, besides not being so 

 much exposed to cannon shot. 



J. W. 

 Bombay, Sept. 21, 1S41. 



THE NELSON COLUMN. 



Sir — On approaching this column from Whitehall, we become sensible of 

 what appears to be a great mistake in the position of it, for while the statue 

 of Charles the First seems to rise in the centre of the street, and in a line 

 with the centre of the portico of the National Gallery, the column is placed 

 quite to the left of that line, so as to appear engaged with the houses near 

 Drummond's Bank, and to rise not in the centre of the street or of the por- 

 tico, but quite to the left of both, thereby producing a most awkward effect. 

 Upon further examination I found this was owing to the architect ha\ing 

 thought it necessary to jilace it in a line perpendicular to the centre of the 

 plane of the portico, whereas had he erected it in a line from the centre of 

 the portico through the statue, this hne though not i/uite perpendicular to 

 the plane of the portico, would have deviated from it in so veri/ stiylit a de- 

 gree, as not to be perceptible to the nicest eya, and would have obviated 

 completely the very glaringly awkward effect of its present position, which, 

 whether viewed as you turn up Whitehall or from the centre of the portico, 

 is equally offensive to the eye, which would have been quite insensilde to the 

 very triHing deviation from the perpendicular in the line above mentioned. 



Aestheticus . 



