18+8,1 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



9S 



that part ; and if the stone he laid with this vein in a vertical di- 

 rection, the block will run the chance of being fractured by a 

 weight, or, if near the surface, it probably may adniit tlie wet. 

 These veins are not like those in tlie Batli stones, which are hard, 

 consisting of crystallized carbonate of lime, and running always 

 in a vertical or inclined direction, and not liable to separation. 

 In general it is considered that the blocks of Caen stone may be 

 placed in construction in any du'ection, except when the wliite 

 veins are perceptible. It is said that the most experienced eye 

 can hardly detect the different qualities of the stone in the block, 

 when once they have been removed from the quarry, as the action 

 of the quarryiiian's tool on the surface hardly offers any indication ; 

 and there is no appreciable difference in the appearance of the 

 ^■anular formation. 



There are in the vicinity of Caen, even to a considerable dis- 

 tance, many beautiful varieties of tliis formation. At Falaise, 

 about 20 miles off, higher up the Orne, is a fine compact stone, 

 mucli harder than tlie Allemagne. Its texture is beautifully equal, 

 and fine grained. Its price is one-third more than that of Caen 

 stone, and, of course the labour upon it is considerably increased. 

 It is well adapted for exposed situations, and is used, I believe, in 

 the quays and dock basin now constructing at Caen. 



I was, of course, anxious to ascertain whether the magnificent 

 and ancient buildings in the city could be relied upon as proofs of 

 the quality of tlie stone in the Allemagne quarries, of which there 

 is a traditional report lianded down from one generation to another, 

 that they are constructed. And, certainly, the lofty pinnacles 

 and spires, and the solid liigli square towers, which rise up in 

 clouds, defying the fury of tlie elements, for many years exposed 

 to storms, hail, rain, snow, and frost, acted upon by all the alterna- 

 tions of heat and cold, wet and dry, present a sliarpness of arris 

 and smoothness of surface, as seen from below, that prove a con- 

 siderable degree of hardness in the stone of which they are con- 

 structed. Less reliance can be placed upon the indications on the 

 parts within reach, for exposed as tliey have been to the Vandal 

 wantonness of the revolutionary phrenzy of destruction, and tlie 

 Cah'anistic zeal of misguided religious feelings, tliere are many of 

 the lower parts broken away and considerably worn. But the 

 attenuated and refined details of some " renaissance" finials, pin- 

 nacles, and flying buttresses, in the lady-chapels and apsidal altar- 

 ends of the churches of S. Pierre and S. Sauveur, and S. Sauveur- 

 le-Marclie of the beginning of the sixteenth century, more 

 minutely enriched and elaborately carved and subdivided than even 

 the most refined details of tlie flamboyant parts near them, are 

 as fresh and sharp as if executed within tlielast fifty years. Time 

 and weather have not liad, on the monuments of Caen, the same 

 corroding hideous influence as on the edifices of Chester, Coventry, 

 or Oxford. Tlie graceful spire of S. Pierre, the summit of which 

 is 250 feet above the market-place, and itself more than 100 feet 

 high, does not appear to be thicker than 9 inches in the lower part, 

 and is reduced, it is said, to + inches thick at top. Tlie immense 

 weiglit and exposed situation do not seem to ha\"e affected it in 

 tlie least degree ; and it may be quoted, if not for size, at all 

 events for its grace, daring construction, and state of preservation, 

 after 540 years' trial, with its sister spire of our own Salisbury, 

 erected at the same period. 



At the same time, I am not prepared to assert whether the stone 

 employed was all taken from the Allemagne, or from some other su- 

 perior quarries ; but the appearance of the stone justifies the tra- 

 dition of its origin, and I know not how to question it. 



CONAVAY TUBULAR BRIDGE. 



Ejcperiments on the Completed Structure. 



Wn are glad to be able to quote from a contemporary an ac- 

 count of the experiments on the tubular bridge just completed, 

 as gi^-en by Mr. Fairbairn himself, in a letter to a friend : — 



" We liave solved an important problem in practical science; and, uespite 

 the prognostication of some eminent mathematicians, tlie wliole of my ex- 

 periments at Millwall have been more than realised. On Wednesday last, 

 the tube was suspended upon temporary piers, 400 feet span ; and with its 

 own weight (1,300 tons), the deflection did not exceed, but was under, 

 8 inches. With 300 tons of loaded trucks, the deflection was increased to 

 11 inches — being, as near as possible, in the ratio of 1 inch to 100 tons of 

 load. The computed breaking weight of the tube is 2,200 tons equally dis- 

 tributed, exclusive of its own weight; and, having its perfect retention of 

 form and great rigidity, I am of opinion that it would sustain 3,000 tons 

 before fracture took place." 



It appears from this account, that the deflection under a load of 

 300 tons, is less than one foot — an amount which Mr. Fairbairn 



considers so small as to demonstrate the successful issue of the 

 undertaking in which Mr. Stephenson, with the able co-operation 

 of himself and Mr. Hodgkinson, is engaged. Certainly, wlien we 

 consider the length of tlie structure, the multiplicity and complex- 

 ity of the component parts, and the number of joints and rivets 



the accuracy of adjustment, and the extreme nicety of workman- 

 ship which efl'ect the result stated, must appear wonderful ; and 

 the superintendents of tliis great work, who have concerned them- 

 selves in its minutest details, and therefore have the fullest sense 

 of its difficulties, must naturally estimate this amount of success 

 more highly than comparatively uninterested persons can do. 

 But iron, even of the best quality, is not perfectly elastic; bolts 

 and rivets, though ever so carefully formed, are not mathematically 

 true ; and, therefore, it may reasonably be asked, if the structure 

 sink one foot now, liow much will it sink when the bolts liave been 

 worn, the bolt-hides enlarged, and the plates strained by the wear 

 and tear of six months' railway traffic ? 



It is to be remembered, also, that the dynamical effect on the 

 structure of a load in motion, is much more" than the statical effect 

 of a load at rest. In the case of a jointed structure, of which 

 the elasticity is imperfect, the dynamical strain and deflection 

 would be certainly double the corresponding statical eflect. 



These remarks are not intended as forebodings as to the ulti- 

 mate success of this magnificent undertaking. All that we wish to 

 do is to point out how much of the problem is solved, and how 

 much remains in doubt. Considering the question abstractedly, 

 we cannot deny tlij possibility of making the structure strong 

 enough to bear its load. Theoretically, a tubular bridge may of 

 course be made strong enough to bear any assignable load what- 

 ever — ton after ton of metal miglit he added till the requisite 

 strength would be obtained. For as each ton of metal would be 

 disposed so as to bear something more than its own weight, we 

 should, by continuing the process of increasing the thickness of 

 the plates, arrive ultimately at a point where the strength was suf- 

 ficiently in excess to sustain any load assigned. 



But the question is, not wliether the bridge may be made strong 

 enough, but whether it be made so at the least expense of material. 

 It is to this point our doubts refer. Jlr. Fairbairn says, that his 

 experimental results contradict the conclusions of some eminent 

 mathematicians ; and, except for the laudatory epithet, we should 

 be disposed to tliink that he refers to investigations wliich have, 

 from time to time, appeared in this Journal^ in which alone, we 

 belie\'e, the mathematical principles of tlie tubular bridge have 

 been discussed on an extensive plan. But leaving the personal 

 question, it is enough to explain that we call in question not the 

 effect, but the means ; not the sufficiency of the structure, but its 

 economy. It has been already shown (Vol. IX. for 1846, p. 300), 

 that straight tension rods, proceeding in right lines from high 

 suspension towers to several joints along the tube, would act with 

 the greatest possible efficiency. It is not even now too late to 

 apply tlie suspension rods to the bridge : only let it be by recti- 

 lineal rigid diagonal bars — not by flexible or catenary chains. 

 Comparing equal quantities of metal disposed — first, in increasing 

 the thickness of the tube — secondly, in diagonal bars, acting 

 eitlier as struts beneath the tube, or as tension rods above it, — it 

 has been mathematically demonstrated that the efficiency of the 

 metal may be trebled by the second method. AVere it not dan- 

 gerous to prophesy on a subject so novel and so difficult, we should 

 be inclined to predict that this second method, in one or other 

 of its forms, of diagonal tension rods or diagonal struts, will be 

 found necessary after the structure has been some time in use. 



NOTES OF THE MONTH. 



TAe Tabernacle. — Among the interesting exhibitions now open is that of 

 the Tabernacle of Israel, at 58, PalliVIall. The Rev. R. W. Hartshorn, a 

 clergyman of the University of DubUn, feeling an interest as to the form 

 and structure of the Tabernacle, has had a model made, with all the details 

 elaborately executed, as gohl and silver candlesticks, brass sacrificial instru- 

 ments, and embroidered curtains. The models are two in number, and are 

 executed in strict conformity with the texts in the bible, which describe the 

 arrangement of the original Tabernacle of the Jews. The first of these 

 models represents the Jews encamped in the plain of lloab, with the tribe of 

 Levitts and the Tabernacle in the centre. The tents of Ephvaim are shown 

 in the distance, and afar off the Dead Sea and the mountain range. This is 

 a most interesting tableau. The other model is devoted to the illustratioa 

 of the court of the Tabernacle in greater detail. Here are shown the sixty 

 pillars, the altar of burnt offering, the embroidered curtains, and all the ac- 

 cessaries of the place of worship. Tlie water-vessels are copied from au- 

 thorities in the British Museum ; the pdlars are gilt, the candlesticks and 

 vessels are of gold and silver, and the model of a high priest stands at the 



