1P50.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



827 



that no distortion would then have occun'ed, and valuable results 

 would jirohably have been obtained. Permanence of form niifrht, 

 moreover, he entirely ensured by diaphrai^ms or stops, at intervals, 

 thioufihout the tube, or by stiffening-plates united by angle-iron, 

 as in the bridf^es. Such diaphragms have, indeed, been success- 

 fully ado))ted by Professor Airy in using vvrouglit-iron tubes for 

 the sujiport of astronomical instruments, to which purpose they 

 are peculiarly ai)plieable, on account not only tif their stift'ness, 

 but of their greater freedom from vibration or trenu)r than cast- 

 iron supports. Diaphragms are used in tiie construction of the 

 wrought-iron polar axes of the large equatorial telescope in the 

 Observatory of Liverpool, which are formed of two semi-elliptical 

 boiler-plate tubes, of e.xquisite workmanship. 



"Circular wrought-iron tubes, of considei-able thickness, and of 

 magnificent dimensions, retained in shape by stops, are also being 

 used by Mr. Brunei in the construction of a bridge over the ^\'ye, 

 at Chepstow in South Wales. These tubes are, however, not 

 strained transversely, except in supporting their own weight during 

 the process of erection, and for this purpose it is intended to render 

 them temporarily more rigid by cambering them to a slight extent 

 by tie-rods along the bottom. They are 305 feet long, 9 feet dia- 

 meter, and g inch thick ; and are employed as struts, or pillars, to 

 resist the horizontal strain of the suspension links which support 

 the wrought-iron girders of which the bridge is composed. By 

 these means, without the usual tie-chains of a suspension-bridge, 

 the lofty towers are relieved from all lateral strain. 



"The total span of this bridge is 300 feet, which is the length of 

 the circular tube employed as a strut; a chain, consisting of three 

 straight links, suspended from this strut, divides the span into 

 three equal portions of 100 feet each. The beam carrying the 

 roadway is thus a continuous beam, 300 feet long, supported at each 

 end and at two points in its length. The circular tubes are 

 supported on cast-iron standards. 



"Circular tubes, 100 feet high, were also at one time proposed as 

 supports for the platforms in constructing the abutment-tubes of 

 the Britannia Bridge. 



"The round tube, as proposed for the bridge itself, if suspended 

 in chains, and merely applied as a means of ensuring a rigid plat- 

 form, would, if constructed with thick plates, properly united, 

 have formed a most efficient structure, offering but little resistance 

 to the wind, and having equal rigidity in every direction; while 

 an elliptical tube of the depth necessary for the Britannia Bridge, 

 and well retained in shape, possesses several important advantages 

 as an independent beam. The curved plates of the top are well 

 adapted for resisting compression, and for throwing otf the wet, 

 while the heavy riveting necessary for uniting the sides with the 

 top and bottom in a rectang\ilar tube is entirely obviated; although 

 there are other more important practical advantages in favour of 

 the rectangular form. 



"We have many instances, in the vegetable kingdom, of the ex- 

 treme rigidity and strength of circular tubes: the stems of the 

 grass tribe generally are remarkable for their lightness and 

 strength ; the common wheat-straw and the river reed are familiar 

 examples in our own climate; but in the tropics the gigantic stems 

 of the bamboo and other grasses tower sixty feet above the jungle, 

 and are extensively employed as beams for covering buildings, and 

 even, in some cases, as the transverse bearers of light suspension 

 bridges. The angler's bamboo rod is the most perfect of tubular 

 beams. Tapered off in proportion to the strain, its salicious coat 

 (as in all the grasses) defies compression, while it is internally lined 

 with woody fibre to resist extension in every direction ; its strength, 

 lightness, and stiffness, are thus equally marvellous; and we cannot 

 fail to be struck with the provision of diaphragms throughout the 

 whole tribe, to preserve the circular form, which addition would 

 certainly have much modified the results obtained from thin circular 

 and elliptical tubes of wrought-iron. 



This illustration from the vegetable kingdom, is only one among 

 many examples of the writer's happy power of treatment, and will 

 enforce upon our readers the importance of the study of animal 

 mechanics, which so far as we are aware is not taught in any en- 

 gineering college. 



In reference to the ultimate length to which it is possible to 

 carry the tubular bridge, Mr. Edwin Clark has several remarks, 

 which we think will prove of interest to our readers, and in the 

 discussion of which Mr. Clark again alludes to the works of nature. 



"Again, if we make a tube similar to another, increasing every 

 dimension except thickness, the absolute strength will be directly 

 as the increase, that is to say, another tube twice the length, depth, 

 and breadth of the Conway Bridge, but of the same thickness, would 



be just twice as strong; it would, however, be four times as heavy, 

 and hence have four times the strain from its own weight, anil 

 would, therefore, soon come to a limit at which it would break 

 itself. 



"Tills is evident by considering that with tubes of similar section, 

 in which the thickness is not altered, the sectional area will bo 

 simply as the increase, and not as the square of the increase; thii 

 strength will therefore be simply as the lineal dimensions, instead 

 of as their square. 



"But if we increase a tube in depth, and length, and width, and 

 preserve its sectional area constant, that is, if the plates are 

 thinner in the same proportion as the tube is enlarged, then the 

 absolute sti-ength of the enlarged tube ad infinitum will be the 

 same as that of the first. So that by keeping the same sectional 

 area as at Conway, and enlarging in the same proportions the 

 length, breadth, and depth, we may make a tube of any length, 

 equally strong, theoretically, with the Conway Tube. For tho 

 strength is directly as the sectional area into the depth, and in- 

 versely as the length, and the sectional area being constant, as 



well as the ratio 



depth 



the strength will also be constant; but 



the weight of the tube, and hence the strain from its own weight, 

 would increase as the length; and, consequently, if we suppose 

 the strain to be five tons per square inch at present in the Conway 

 Tube, another tube of the same sectional area, and of three-and-a- 

 half times the same length, breadth, and depth, would fail by its 

 own weight. Such a tube would be 1400 feet long, and no increase 

 of thickness would make such a tube bear more than its weight. 



"We have already alluded to the strength of the bamboo as an 

 instructive natural example of the strength of a circular tube. The 

 bones of animals are oval, the depth being always in the direction 

 of the transverse strain. But the more special province of the 

 bones appears to be their action as pillars, or struts, in forming im- 

 moveable fulcra for the reaction of the muscles; and since any 

 yielding would involve a great increase of motion in the tnuscl.'! 

 itself, we find bone among the most imcompressible of known 

 substances. 



"The square form of stem characterises a very extensive natural 

 family of plants — the labiate tribe, of which the beautiful dead 

 nettle of the hedgerows is an example; though it is dillicult to 

 assign any mechanical reason for this peculiarity, which appears 

 rather to be typical of the general developement of these plants. 

 But in the feather-bearing part of the ordinary quill we have a 

 most remarkable example of the strength of the rectangular form. 

 Here, again, every dimension is tapered down in proportion to Xh^. 

 strain, with an accuracy defying all analysis; the extended and 

 compressed portions are composed of a horny substance of prodi- 

 gious strength, though extremely light and elastic. The beam is 

 not hollow, but to preserve its form it is filled with a pithy sub- 

 stance which replaces the clumsy gusset pieces and angle-irons of 

 the tube without interfering with its pliability; the square shaft is 

 peculiarly available for the attachment of the deep vanes which 

 form the feather; and as the angular form would lacerate its active 

 bearer, an exquisite transition to the circular quill at the base is 

 another striking emblem of perfection. The imitation of such 

 mechanics, so wonderfully ada])ted to such a medium, appears hope- 

 less; but we are indebted to the flying philosopher, if his attempt 

 only calls attention to such design, and induces us instructively to 

 contemplate the beauty of a feather." 



REMARKS ON SPIRIT-LEVEL ADJUSTMENTS. 



There are some misapprehensions prevalent affecting the manipu- 

 lation required for properly adjusting the spirit-level, and the 

 reasons which occasion it. Such errors, if copied from one text 

 book into another, are likely to mislead some of the profession, 

 who may not have leisure to examine for themselves. 



The object of the adjustments should be to enable us to obtain 

 at any place a straight line of sight, revolving in a plane; this 

 plane to be a tangent to the earth s surface at that place. 



The term optical axis is sufficiently explanatory; the term line of 

 collimation is not so. jNIr. Simms, in his 'Treatise on Levelling,' 

 writes, optical axis, or line of collimation. This description, if in- 

 tended for the old-fashioned Y-level, in which both should coin- 

 cide, would be correct; but is inapplicable to that with fixed teles- 

 cope at present in general use. In the latter the adjusted line of 

 siglit may or may not form an angle with the optical axis of the 

 lenses. Provided the line of sight be parallel to the bubble,. and 



