220 



THE CIVIL ENGINEER AND ArX'IHTECTS JOURNAL. 



[July, 



ograiM d m, tlieii e d and rfc are equivalent to e c, and these three forces 

 are in c(|uilibiium. Thrrefore, cc being the resistance, de is the 

 power, ami dc or c ;«, the pressnre on the centre k, whicli, as it never 

 has intilion, is of no consequence. Taking any point/in be produced, 

 drawy"^' perpendicular to it, meeting eg in gi jyand eg will express 

 the relative virtual velocities of the resistance and po\Ver respectively, 

 andy"^, the ))assivc lateral motion of the line of resistance, — jiassive, 

 I say, for its direction is at right angles to thi^, and it is therefore of 

 no consequence. Hut the triangles /e^, rfcc, are similar, therefore 

 de '. ec '. '. /e : eg, and de- egz=fe- ec, that is, the momenta of 

 the power aid resistance are equal. The same conclusion is due at 

 every olhcr point in the circle b I, An addition to the power will set 

 the engine in motion, which would be uniformly accelerated were it 

 not that the resistance ircreascs with the velocity. However great, 

 then, the power maybe, there will ultimately obtain a uniform motion, 

 when the power and resistance will be in equilibrium, their momenta 

 being equal, as before. Therefore in connecting rod motion, force for 

 force is given and received, and there is no loss essential to that mo- 

 tion. 



The point e of ke moves alternately in a circle. The greater limit 

 of this ang\ilar vibration is a semicircle, in which case ke 7^ ab. The 

 smaller limit is a straight line > an indefinitely small portion'of a circle, 

 its radius A' (, being indefinitely long. This limit is practically exem- 

 plified in engines in which the piston rod is at once jointed to the 

 connecting rod, as in the annexed sketch, fig. 2, of this motion, in 

 • which a 6, and 6 e are the same as in last figure ; e /■ the cross head, 

 bearing perpendicularly on the slide surface I m, parallel to the piston 

 rod A(, evidently in the same way as if bearing round a centre k, in- 

 finitely distant. 



Upon the whole, then, short and long connecting rods on the same 

 length of crank must be equally effective, whatever peculiarities there 

 be. 



I am, Sir, your obedient servant, 



Daniel Clark. 

 Phexnix Iron Works, Glasgow, 

 JujieS, 1841. 



SLOPES IN SIDELONG GROUND. 



Sir — The following formula for "setting out slopes in sidelong 

 ground," requiring the distances to be measured along the ground, 

 and not horizontally, has, for that reason (particularly where the ground 

 is very steep), an advantage over the formula in your last number; 

 should you agree with me in this opinion, you will perhaps find a 

 place for it in your next Journal. 



Let 2 >» = width of the railway =: AB. 



0=2/. of the slopes. 



e= Z of the natural ground. 



h = depth of cutting = C G. 

 Then (w tan (3 + F Z ) = C F. 



sin C F D 



C D = (w tan -\- l>) '.„r-v^ = ('' t^" P + J>) 



cos $ 



C E = (ro tan /3 + A) \ 



sinC EF 

 sin C F D 



= {m tan $ + h) 



sin (/3 -f e) 

 cos 



sin C D F ^ ■" " ' ■■' sin (6 — e) 

 The slopes remaining constant then rs tan will be constant, and 



therefore the angle CDF will also be constant. 



I am, Sir, your's most obediently, 



Manchester, June 8, 1841. 



W. R. 



THE NELSON COLUMN. 



Sir — As the Nelson column rises to view, we become sensible of 

 wli;it appears to be a great mistake in the position of it, and which 

 ought to have been in a line drawn from the centre of the portico of 

 the National Gallery through the Statue of Charles I., which appears 

 as it ought to do in the centre of Whitehall ; whereas the column seen 

 from the same spot, will appear considerably to the right of the statue, 

 and will be engaged with Drummond's Bank, the Admiralty, &c., in- 

 stead of appearing to rise in the centre of the street, and thus pro- 

 ducing a most awkward effect, whether as seen from the centre of the 

 portico, or in approaching it as you come from Whitehall. This might 

 easily have been avoided by placing the column in a line with the 

 statue, which line, though not q.iite perpendicular to the plane of the 

 portico, would have deviated from it in so very slight a degree as not 

 to be jierceptible to the eye, while the present position will produce 

 an elfect so glaringly awkward as at once to strike every beholder. — 

 The mistake of the architect consists in having thought it necessary 

 to place the column in a line perpendicular to the plane of the portico, 

 whereas his object should have been to make the column appear to 

 rise in the centre of the street, as seen from the portico which could 

 have been done by the very slight and imperceptible deviation from 

 the perpendicular above mentioned. 



I am. Sir, your's, 



^STHETICUS. 



ON THE THEORY OF BARS. 



" Lorsque I'homme s'ecarte de la vraie cause d'un objet quelconque, 

 il doit se considerer dans les tenebres, et il est force de chercher des 

 arguments absurdes, dans lesquels il se perd, ce qui fait que les 

 sciences deviennent ridicules dans I'opinion du vulgaire." — Cuvier on 

 marine deposits. 



Sir — Pursuant to the notice I gave in the last number of your valuable 

 Journal, I take leave to send you for insertion the following observa- 

 tions on a "New Theory of Bars, &c., by Mr. Brooks." 



The importance of the subject to this great naval and nautical na- 

 tion, and to the maritime commerce of the world, should admonish us 

 to pursue the investigation of this matter with the most cautious and 

 serious consideration, for as it is well observed in the quotation at the 

 head of Mr. Brooks' treatise, "our errors in this matter are of more 

 importance than in mere objects of taste, luxury, or pleasure, because 

 they will ever result in injury, or in the loss of some previous advan- 

 tage." Let us also bear in mind Cuvier's reproof quoted above. 



It does not appear requisite that I should refer to the many theories 

 quoted by Mr. B., the controversy so prevalent at present, and in past 

 times, in the scientific world on the subject of bars, demonstrates that 

 it has not received that attention and examination which can lead to a 

 right conclusion as to their cause, and what are the most eligible means 

 to obviate the many evils incident to their existence ; but I do pre- 

 sume that my subsequent remarks, based on facts and practical obser- 

 vations, will prove, that if the desideratum has not previously been 

 developed, Mr, B. has not reflected any new light on a subject hitherto 

 by many supposed to be enveloped in darkness. 



It appears apposite to notice that Major Rennel, quoted by Mr. B., 

 p. 1 and 2, states, " that mud and sand suspended in the waters," (i.e. 

 the egress waters) "during their motion are deposited when that mo- 

 tion ceases, or rather they are gradually deposited as the current 

 slackens, according to the gravity of the substance suspended ;" and 

 the late Mr. Telford gave a similar exposition. I did not expect in 

 this age of the world, any one would reject such an evident and irre- 

 futable fact, a principle ever in operation during the discharge of the 

 egress tides, or currents ; but Mr. B. p. 4, says, "1 venture to submit, that 

 it is insufficient," (/. e. the Major's thesis) "to account for the forma- 

 tion of bars, because the operation described (the deposit), as produc- 

 ing the latter (the bar), takes place in all rivers, in a greater or lesser 

 degree, and in those which although their waters are abundantly loaded 

 with sand or mud, are nevertheless free from bar." Mr. B. therefore 

 disputes the accuracy of the Major's deduction, because it is the result 

 of a partial, and not of a general law ; why, Mr. B. has endeavoured to 

 rest his entire case on local and partial data, and neglected to observe 

 general principles. 



That all rivers, harbours, bays, estuaries, &c., where the waters pass 

 with a velocity sufficient to hold matter in suspension, have bedn of 

 sand, &c., is quite correct; but where the receding waters do not re- 

 turn or run out into the ocean with a force adequate to disturb the 

 deposit that occurred during the qniescent state of the waters, as 

 described by Major Rennel, there cer» •inly «o bar or exterior accumula- 



