1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



117 



to the circumference nuiltiplied by 3-141. Hence the ratio of the cen- 

 tral force to the force in the direction of the circle, or the moving power, is 

 as the product of the nnmhtr of rifolutions in a second hi/ 3-1-tl is to unit. 

 That is, if tiiere be two entire revohitions in a second, wliatever be 

 the weight of the body or its distance from the centre, the ratio of tlie 

 centrifugal force to the moving power would be as 3-Ulx2 is to unit, 

 or as six to one, nearlv ; and with eight revolutions in a second the 

 ratio is as 3'141 X b to unit, or as twenty five to one. And since "the 

 velocity of rotation is almost unlimited,"* if a fly-wheel, similar to the 

 one described above, were revolved at the rate of twelve hundred re- 

 volutions in a minute, the excited or centrifugal force in the rim would 

 be equal to sixty-two and a half times the amount of power employed 

 to give the requisite velocity, some deduction being made for friction 

 and atmospheric resistance. 



ON THE POWER OF FLUIDS IN MOTION. 



In Silliman's American Journal for January last appears the follow- 

 ing abstract of a paper read at the American Philosophical Society, 

 " On a new Principle in regard to the power of Fluids in Motion to 

 produce Rupture of the Vessels which contain them, and on the Dis- 

 tinction between Accumulative and Instantaneous Pressures ; by 

 Charles Bonnycastle, Professor of Mathematics in the University of 

 Virginia." 



Mr. Bonnycastle's investigation was suggested by a paper read by 

 Dr. Hare, and printed in the Transactions of the Society, entitled 

 "On the Collapse of a Reservoir, whilst apparently subject within to 

 great Pressure from a Head of Water." Dr. Hare pointed out the 

 circumstances attendant upon this curious occurrence, and showed how 

 the vessel might have been momentarily relieved from the pressure 

 of the water within, so as to make that of the surrounding air efficient 

 in producing the collapse. The principal object of Mr. Bonnycastle's 

 paper is to investigate the precise nature and degree of the forces 

 brought into action in this and similar cases. 



The results at which Mr. Bonnycastle arrived, are stated by him as 

 follows ; — 



1. It is convenient to distinguish between accumulative and instan- 

 taneous loads, or between tliose which are gradually increased until 

 the deflection due to the ultimate load is obtained, and those which 

 commence in full efficacy from the initial position of the support. 



2. Within the limits of perfect elasticity, instantaneous pressure 

 produces twice the effect of that which is accumulative, whether the 

 result be to produce deflection or fracture. 



3. In regard to supports perfectly elastic in one direction, and per- 

 fectly flexible in the other, instantaneous action, at right angles to the 

 axis of elasticity, produces a deflection which is to that of accumu- 

 lative action as V'l to 1, whilst the tendencies to fracture are as 4 to 1. 

 But should any case occur when the law of elasticity follows an ex- 

 tremely high power of the deflection, then the singular result will 

 follow, that the deflections are the same, whether the force be exerted 

 from the initial state or the state of load, but that the tendency to 

 fracture will be immensely greater in the former case than in the 

 latter. 



4. In producing the fracture of natural substances, which all depart 

 from the law of perfect elasticity as we approach the limit of fracture, 

 the ratio of the effect of instantaneous and accumulative action will 

 vary with the nature of the substance, never being less, for elastic 

 bodies, than 2 to 1, nor for flexible than 4 to 1, and more usually ap- 

 proaching 3 or 4 to 1 for the former case, and 5 or to 1 for the 

 latter. 



5. Let a vase or conduit be acted upon by a load which is alone 

 suflicient to break it, and let this load be partly balanced by a small 

 exterior force ; should the great interior force suddenly cease, the 

 small exterior action may crush the vase or conduit inward ; its energy 

 in such case being the sum of the interior and exterior forces. 



6. Should the interior force be a vibration of the kind already ex- 

 plained, and should the exterior action be extremely feeble, and act 

 on a very great mass, this extremely feeble action may crush the vase 

 inward, with a power that shall exceed in any degree the enormous 

 action of the interior or explosive vibration. The comparison of the 

 interior and exterior actions is best eflTected in this case, by finding 

 the modulus of elasticity of a material spring that shall coincide most 

 nearly in effect with the interior tremor. For putting e and e' re- 



pectively for the modulus of the spring and of the support, and it and 

 ' for the deflections resulting from the tremor acting alone, and the 



reaction as it does act, we have — = . / -, or, in other words, the 

 • Fisher's Nat. Philos. 



deflection produced by the reaction, is to the deflection that would be 

 produced by the interior tremor alone, in the inverse proportion of the 

 square roots of the moduli of tremor and support. 



7. Combining w hat is here said with the known laws of fluids mov- 

 ing in pipes, and whereby they necessarily produce hydraulic shocks, 

 it follows, that any vessel connected with such a train of pipes, and 

 plunged at some little depth in a considerable mass of water, or other 

 heavy fluid, will occasionally be subject to a crushing and exterior 

 force vastly greater than the interior strain due to the constant head 

 of fluid. 



In illustration of the principles thus developed, Mr. Bonnycastle 

 details some experiments, and mentions a phenomenon which occurred 

 under his own notice, and is analogous to the one described by Dr. 

 Hare. In making experiments on the propagation of sound through 

 water, he had occasion to cause an explosion of gunpowder within a 

 hollow metallic cylinder, open at the lower end, and immersed under 

 the liquid; and, although the strength of the cylinder was abundantly 

 sufficient to bear the statical pressure of the surrounding water, he 

 found it crushed inward after the explosion. 



ROMAN ARCHITECTURE. 



[We have heretofore had occasion to speak in praise of several 

 articles on architecture which have appeared in the "Penny Encyclo- 

 pffidia," and have given several extracts; we have now much pleasure 

 in making some additional extracts from a very able article on " Ro- 

 man Architecture," which appeared in one of the recent numbers.] 



With regard merely to the orders, Roman architecture presents 

 chiefly a corruption of the Doric and Ionic, for it may claim the Co- 

 rinthian as almost entirely its own, the Roman examples of that order 

 being not only numerous and varied, but at the same time exceedingly 

 different in character from the almost solitary specimen of one with 

 foliaged capitals which occurs in a Grecian building. But even as re- 

 gards the application of the orders, there is a wide difference between 

 the two styles ; in the Roman they are frequently employed as mere 

 decoration, the columns being engaged or attached to the walls, or in 

 some cases (as that of triumphal arches) though the columns are in- 

 sulated and advanced from the structure, they are in a manner de- 

 tached from it, inasmuch as they do not support its general entablature, 

 but merely projecting portions of it. Nor are these the only dift'e- 

 rences, for besides the frequent employment of pilasters as substitutes 

 for columns — that is, as constituting the order without columns — the 

 practice of supercolnmniation, or raising one order upon another, was 

 by no means uncommon ; a practice that was indeed a matter of ne- 

 cessity in such enormous edifices as the Colosseum, if columns were 

 to be employed at all. From all this it will be evident that, as regards 

 the orders alone, there is a very marked difference between Roman 

 and Grecian architecture ; yet such difterence is by no means the whole, 

 the two styles being almost opposites in nearly every respect. If there 

 were no other distinction between them, that arising from the arch, 

 and diverse applications of its principles to vaults and domes, would 

 be a very material one ; but we also meet with a variety and com- 

 plexity in Roman buildings which does not occur in those of Greece. 

 The only instance that we are acquainted with in Grecian architecture, 

 of anything like grouping or combination of building, is that of the 

 Erechtheion, or triple temple on the Acropolis of Athens. With this 

 exception, Greek temples were merely simple parallelograms, dirt'er- 

 ing from each other as to plan only in the number and disposition of 

 the columns around the cella ; consequently, however beautiful when 

 considered separately, a very great monotony prevailed in that class 

 of buildings, at least, in which the forms were so limited and fixed as 

 to preclude any fresh combinations, or anything approaching to what 

 is understood by composition. 



By the adoption of the circular form in their plans, whether for the 

 whole or parts of a building, the Romans introduced an important ele- 

 ment of variety into architectural design ; especially when we consider 

 that to such shape in the ground plan is to be ascribed the origin of 

 the tholus, or concave dome, which harmonizes so beautifully with all 

 the rest, and renders the rotunda-shape at once the most picturesque 

 and the most complete for internal effect, — that in which both unity 

 and variety are thoroughly combined. 'The Pantheon alone would 

 suffice to convince us that the Romans were not mere copyists, and 

 that if as such they deteriorated the Greek orders, they also added 

 much to the art, and greatly extended its powers by new appliances. 

 As regards its exterior, the Pantheon presents what is certainly a 

 strikingly picturesque (and what we consider to be also a consistent 

 and appropriate, because a well-motived) combination, namely, of a 

 rectangular mass projecting from a larger circular one. In that exam 

 pie the body of the edifice, or rotunda itself, has no columns exter' 



