1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



279 



mental and decorative paintings, and under the direction of Mr. Latilla and 

 other artists has produced specimens of each. Mr. Simpson has, in some 

 instances, adopted the ground of lirae and marble dust, according to the plan 

 of the ancients, on which he has succeeded in fixing the colours, and pre- 

 serving their iutensity and brilliancy so well that even a copious and repeated 

 washing has not removed them, nor materially diminished their effect. Still, 

 he has of course been confined, on this ground, to the use of such pigments 

 as are not injured by lime. But the greatest success has attended tlie use of 

 the beautifully white and hard cement, known as Keene's cement, n^ manu- 

 factured by Messrs. White and Sons of Millbank, on which, when in a moist 

 state, the colours are easily applied, and become, when dry, perfectly fixed ; 

 and it possesses the farther great advantages of admitting of the highest 

 degree of finish, and allowing the use of those pigments which will not bear 

 the mixture of lime. These specimens have been repeatedly washed with 

 soap and water. The painting in encaustic and a tempera (which Mr. Simp- 

 son has been enabled to render permanent) on Keene's cement, when dry 

 and even polished, has proved quite satisfactory ; as the brilliancy and 

 clearness of the tints, contrasted and thrown out by the whiteness of the sur- 

 face, produces an effect superior to any other ornamental painting yet at- 

 tempted. No doubt other materials will be introduced, but it is those' above 

 described to which Mr. Simpson's attention has as yet been principally 

 directed. 



REMARKS ON THB " PONT DU CARROUSEL," PARIS.* 



Sir — I beg to rectify' an incorrect dimension contained in your number 

 for July, relative to the size of the centre arch of the Pont du Carrousel in 

 Paris, and will, with your permission, take advantage of this opportunity to 

 offer some few observations on the construction of this bridge, erected by 

 me, according to the plans of M. Polonceau, whose merit as an engineer of 

 high standing is considerably enhanced by the conception of this very elegant 

 structure. 



The length of the chord line of the three arches is the same, viz., 156 ft. 

 6 in. intrados, and the rise or versed sine is 16 ft. English measure, being 

 full .[^ of the chord. 



The arch is an arc of a circle, whose radius intrados is 198 ft. 5|, and 

 forms between the radii passing through the extremities of tlie chord line an 

 angle of 46° 26'. The depth of the voussoir, or tubular rib, including the 

 flanches, is 2 ft. 10|, and its breadth across the middle is equal to half its 

 depth. Each tubular rib is composed of 23 pieces, having 11 lengths of 

 about 14 ft. 9 in. on one side of the laminated wooden arc, whereas there 

 are on the other side only 10 whole lengths and 2 half lengths, these half 

 lengths being placed against the piers, in order to cross the joints so as to 

 obtain greater rigidity, which by this combination became very considerable. 



The wooden arc, which is composed of good, well-seasoned fir, having 

 been erected, greatly facilitated the work, for once cut to its proper shape, 

 so as to leave a small space between the sides of the wood and the metal, the 

 half voussoirs had only to be appHed thereto, bolted together and forced up 

 into their proper place by means of struts and wedges, after which keys 

 were fitted into grooves reserved for the purpose at the extremity of each 

 portion of the rib ; and when all were ready they were gently forced down 

 to a bearing, so as not to displace or deform the arc, beginning at the two 

 two piers, and terminating at the centre, where the last key tightened the 

 whole ; and so great was the power of these keys, that with a light hammer 

 I could lift the whole arc from the struts, or lov.'er it again with the greatest 

 ease imaginable. These keys enal)led us, therefore, to set the ribs with great 

 precision, and almost simultaneously, which circumstance is of considerable 

 importance in the construction of an iron bridge, on account of the incon- 

 venient effect produced by expansion and contraction of the metal. 



The hollow rib offers considerable advantage over the flat rib, on account 

 of its rigidity, which is so great as to be capable of resisting all lateral strain 

 with the aid of very little cross framing, or horizontal bracing. 



There exists, In the construction of this bridge a peculiarity which has 

 probably not been generally observed, although we were afraid, previous to 

 the work being finished, that it would produce a very unpleasing eft'ect. The 

 evil was, however, unavoidable, as it arose from the circumstance that the 

 Administration des Fonts et Chaussees required a certain height under the 

 centre arch at high water, and the Prefect de la Seine would not on the other 

 hand, consent to the elevation of the roadway on the quay above a certain 

 height. It became, therefore, requisite to inchne the chord line of the two 

 side arches, and although there is in each a fall of 2 ft. 8 in. towards the 

 quay, the eflFect is scarcely perceptible, unless when the water in the river 

 rises nearly to the spring of the arch, which only takes place in the rainy 

 season. 



By the above plan the road over the bridge forms a curve instead of a 

 level, and probably gave rise to the idea that the centre arch was of greater 

 span than that of the other two. The curve of the road was still further 

 increased, so as to obtain tlie level of the quay, by dimmishing the diameter 

 of the circular bearing rings against the buttress to 1 1 ft., whereas the 

 diameter of those next the pier is 12 ft. 5|. 



Great attention was paid to the form of section of the tubular rib, in order 



'' Drauings of this bridge are given in the Jouniul, Vol. U. p. 79. 



to avoid any bad effect that might result from contraction in the casting, and 

 such care was taken in moulding that very little remained to be done to the 

 pieces when they came out of the sand. The tubular ribs were cast and faced 

 at the Forges et Fonderies de Fourchambault, under the management of M. 

 Emile JIartin, who paid very particular attention both to the quality of metal 

 employed, and to the moulding of the pieces. 



The general allowance for contraction of cast iron in cooling is yi^, and 

 this was found to be as near as possible the case. But the greatest difficulty 

 to overcome was the tendency of these voussoirs to bend as they cooled, and 

 that in consequence of the outer edges of the flanches cooling faster than 

 the centre part, because it is well known that the slower the cooling process 

 is carried on, the greater will be the contraction of the metal. To obviate 

 this it was found requisite to curve the model itself about J of an inch in 

 the length of 14 ft. 9 in. 



The circular rings and other cast iron pieces were cast partly in Paris, but 

 the greatest portion in Normandy: with the exception of the facing of the 

 voussoirs or tubular ribs, all the fitting-up was done on the spot. 



The object of having these circular supports for the roadway was to obtain 

 a certain degree of elasticity, sufticient to preserve the tubular ribs against 

 the vibration of the road when carriages heavily laden are passing on the 

 bridge ; and this desideratum was most effectually attained, for I have fre- 

 quently examined the effect, and have invariably found that when under the 

 influence of a heavy load, and while the road was vibrating considerably, 

 that, owing to the elasticity of these rings, the tubular ribs were perfectU 

 steady. I shall be happy to add some further remarks on this subject, if you 

 consider this paper to be worth inserting in your useful Journal. 



I am, Sir, 



Your obedient servant, 



London, July 19, 1842. H. II. Edwards. 



[We shall, with much pleasure, receive any other communication Mr. 

 Edwards may forward us. — Editor.] 



MR. CHARLES WYE WILLIAMS' BOILER PROJECTS. 



Sir— I have just been perusing a table of " Experiments on Furnaces and 

 Steam Boilers," submitted by Mr. Charles Wye Williams to the British As- 

 sociation, and it is with unfeigned regret I observe the valuable time of the 

 Association is occupied with such matter. The ostensible object of this table 

 is to show the superiority of IMr. Williams's mode of effecting combustion 

 over the plan usually adopted, hut never was a table made less calculated to 

 accomplish this object, and more likely to destroy the last vestige of profes- 

 sional confidence reposed in its author. The use of coal to steam engine 

 boilers is the generation of steam, and I should imagine that even Mr. 

 Williams himself cannot deny that that mode of combustion is the best which 

 practically is found to generate the greatest quantity of steam with the least 

 expence ; and Mr. Williams may, when he first has leisure, begin to convince 

 himself that he will not be benefitted by telling steam-vessel proprietors (to 

 whom economy of fuel is of the greatest importance) that he has found a 

 means of increasing the heat of flues, if he cannot also tell them that the 

 consequence of such increase of heat is (and should not lie) an increased 

 evaporation of water by a given quantity of fuel. Mr. Williams, in his ten 

 deductions from his two tables, has under consideration the quantity of 

 water evaporated per unit of coal, economy of time, as he calls it, and the 

 perfection of combustion. In his first deduction he says, that " the quantity 

 of watej evaporated by any given weight of fuel furnishes no test of the 

 quantity evaporated in any given time." In the second, that " the quantity 

 of water evaporated by each pound of coal is a very inadequate test of the 

 quantity of heat given out by such coal." In the third, that " as the weight 

 of water evaporated per pound of coal bears no jiroportion to the weight of 

 water evaporated per hour, economy of fuel may be the reverse of economy 

 of time." In the fourth, that " the area of the furnace has no necessary 

 connexion with the heat generating power of the coal." In the seventh, 

 that ■' the temperature of the products escaping by the chimney will be in 

 the ratio of the heat in the flues, and both in the ratio of the r.iteof combus- 

 tion and the quantity of heat generated," and so on through his other deduc- 

 tions. Now, in the name of all the smoke-preventing schemers of the pre- 

 sent day, what has any one or all of these deductions to do with the supe- 

 riority of Williams' furnace over tlie common one, if Mr. Williams can tell, I 

 shall be much obliged to him, for so far as I can sec, they are of use only 

 in mistifying the subject, and enveloping it still more thickly in dense clouds 

 of smoke. The mode of testing the economy of fuel with any kind of fur- 

 nace or combustion, is of the simplest description, and to all schemers there 

 is only one course open : — let them find how much water can be evaporated 

 per unit of coal, used in the common way, with any boiler, and let them find 

 how much fuel used in their way is required to evaporate the same quantity 

 of water in the same time, with the same boiler unaltered, except in the 

 furnace. Such is the way in which they will meet the case in |iractice, and 

 such is the way in which they must treat it to jirocnrc the coiifidence of 

 steam-vessel proprietors, and I may perhaps be allowed to add such is the 

 way in which Mr. WilUams' table does not treat it. 



I am, Sir, your obedient servant, 



A. 



2 Q 2 



