152 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Mav, 



thence throu);h the metallic flue of the bciilcr, into tlie hricked 

 flue at the front of the hoilcr, where the current divides itself, 

 and pusses through two brick flues, arranijed one on each side of 

 the boiler, into the ordinary danijier-flue, wlience it passes into the 

 chinuiev. 



Hy examininj; the diairiam it will be at once obvious that, as the 

 flame and heat flow continuously over the bridge, and throujjh 

 the flues beneath, and at the end of the boiler, the brick-work 

 of those flues must imbibe so much caloric as to become red- 

 hot. Further, that should any fracture or rent of the boiler take 

 jilace, so as to allow of a portion of the heated water to flow 

 thence, on that red-hot brick-work, enormous volumes of steam 

 must be instiintmieously generated — capable, by that agency alone, 

 of producing all those disastrous effects to which we liave referred. 

 In a wagon-shaped boiler, however, the effects, under the same 

 ))ressure, will be greater than with the Smeaton or egg-shaped 

 boiler, by consequence of the greater area exposed to the pres- 

 sure. 



It is well-known, naturally, that all power is transmitted in a 

 right line, aiul that the operatiim of compound forces is necessary 

 to produce any deviation from it. If, therefore, the steam thus 

 suddenly evolved from the heated water by its coming into con- 

 tact with the red-hot brick-work of the flue, could, at the same 

 instant of time, be equally diff"used under every portion of the 

 bottom only of the boiler, and act on every part of it with equal 

 intensity of pressure, there is not a question that the boiler %vould 

 be projected vertically into the air. But such range of flight is 

 liarely likely to take place. Even if the steam could be equally 

 diffused under every portion of the bottom part of the biuler at 

 the same time, the tire-grate, owing to the interstices between the 

 fire-bars, does not present that firm base for the steam to act 

 against as is presented by the solid mass of brickwork behind the 

 bridge. Therefore, the great iirobaldlity is, should any such frac- 

 ture take place, either over the bridge," or on the right-hand side 

 of it, or over the flue, to however small an extent, the boiler will 

 be projected through the air in an oblique direction; and the 

 deviation from the vertical line will be greater or less, accordingly 

 as the fracture takes place nearer to, or farther from, the bridge, 

 towards the chimney. The greater accession of heat, also, that 

 may be imparted to the water by such brick-work, and the action 

 of the pressure on the end of the boiler, at the flue will tend, 

 still further, to the oblique direction we have stated. 



The observations we have thus made are of great practical im- 

 portance. Hitherto, from the awful effects of such explosi(Mis, the 

 minds of practical and thinking men liave been devoted more to a 

 search after some unexplained cause for increased jiroduc'tion of 

 pressure within the biuler, rather than to an elucidation of thesimple 

 one we have developed, and by which, in our opinion, most of 

 those catastrophes are produced. High-pressure steam is not 

 indispensably necessary to an explosion. Low-pressure steam is 

 amply commensurate to the end. W^e have shown this by our 

 remarks on the explosions that took place of the two Smeaton or 

 egg-shaped boilers; and we can confirm or strengthen the state- 

 ments by adding, that we were present shortly after, and witnessed 



the effects that had been produced by an explosion of a wagon- 

 shaped boiler. It had been worked, customarily, at from 7 to 9lb. 

 pressure on the square inch, and there was reason to believe that 

 that pressure had not been exceeded at that time. It was in con- 

 nection with two other boilers, neither of which had exploded. In 

 short, it is to neglect, superinduced by a false notion of securitv, that 

 such explosions may, in general, be attributed. Engine proprie- 

 tors and engine-tenters, not having been aware of the danger, 

 have, until now, been indifferent, comparatively, as to any defective 

 state of a low-pressure boiler. How freipiently do we see such, 

 while working, leaking badly; but not sufticiently to produce an 

 explosion. How often do we hear that the engine-tenter, even 

 with the sanction of his emjiloyer, has had recourse to some paltry 

 patchwork of a contrivance, to prevent a defective boiler from 

 e.xtinguishing the fire. Had the danger we have pointed out been 

 known, would such things have been allowed to exist.'' Both 

 the engine-proprietor and engine-tenter would, for their own 

 interests, have been averse to it. It cannot, therefore, be too 

 well-known that steam-engine boilers are, by neglect, (piite as 

 liable to be exploded when worked at low rates of pressure as at 

 high. Nor can the reasons we have thus assigned as the cause of 

 such explosions be too widely disseminated. 



IMPACT OF ELASTIC BEAMS. 



On the Impact of Elastic Beams: Abstract of a paper read 

 before the Cambridge Philosophical Society, Dec. 10, 1849. By 

 HoMEttSHAM Cox, B.A. Jesus College. [From the Philosophical 

 Magazine.] 



" Among the experiments instituted by the Royal Commission, 

 appointed to inquire respecting the use of iron in railway struc- 

 tures, was a series relating to impact on beams. These experi- 

 ments were undertaken by Professor Hodgkinson. and were 

 conducted in tlie following manner. The two ends of the beam 

 were fixed in a horizontal position, and the blow was given against 

 one of its vertical sides, in a horizontal direction. The instru- 

 ment for giving the blow was a heavy iron ball, hanging down, 

 when at rest, from a point of suspension vertically above the centre 

 of the beam. The ball was raised through different arcs, and 

 after descending by its own gravity struck the beam. The deflec- 

 tion corresponding to different arcs of descent were carefully 

 noted by a graduated scale. The object of the present paper is to 

 show that the results might have been predicted by known theo- 

 retical principles with considerable accuracy. The problem is 

 divided into two parts: 1st — to estimate the amount of velocity 

 lost by the ball at the first instant of collision ; 2nd — to ascertain 

 the effect of the elastic forces of the beam in destroying the 

 ins viva which the whole system has immediately after colli- 

 sion. 



"In the first part of the investigation a general formula, derived 

 from the combination of D'Alembert's principle and the principle 

 of A'ertical Velocities, is given for the motion of any material system 

 subject to impact. The requisite geometrical condition required for 

 the application of this general formula, is obtained by the assump- 

 tion that immediately after impact, the form of the beam is a 

 gradual and tolerably unif(u-m curve, such as, for example, the 

 elastic curve of eipiilibrium. In this way it is determined that 

 about one-half the inertia of the beam is effectively applied at the 

 instant of collision to retard the ball. 



" The vis viva of the w hole system thus computed, is destroyed by 

 the elastic forces of the beam develoi)ed by deflection. These, in 

 the second part of the problem, are assumed to vary as the amount 

 of the central deflection. By the principle of vis viva a formula 

 is easily obtained, connecting the total deflection with the vis viva 

 of the system immediately after collision. 



" Tables are given, in which the theoretical and experimental 

 results are compared. The correspondence is of the closest and 

 most satisfactory nature. Indeed, the theoretical results generally 

 differ less from the mean of several experiments, than those expe- 

 riments differ among themselves. Both in the theoretical and 

 experimental inquiries, every possible variation of the elements of 

 the investigation — the relative masses of the beam and ball — the 

 velocity of the latter — the rigidity and dimensions of the former — 

 have been included." 



