1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



.319 



The intensity of the iintlulations of gaseous fluids, as well as that of the 

 air, is proportional to the amplitude of the oscillations, or more properly to 

 the square of the amplitude. 



A wave of light striking the retina must create a similar vihratory motion 

 in the nerves of the retina, hecause the velocity of the molecular movement 

 of the nerves depends upon the force with which they have been struck by 

 the original wave, and if this velocity could be measured, it would show at 

 the same time the intensity of light. 



It is scarcely possible to obtain a direct accurate measurement of this velo- 

 city, but if the time during which the viliratory motion of the nerves ceases, 

 be ascertained, the velocity of the vibrating molecules, and therefore the in- 

 tensity of light, may be determined ; because the duration of an impression 

 on the retina is dependent on the resistance w hich the molecules of the nerves 

 oppose to every force striking them ; lint as tliis resistance of the nerves in- 

 creases as the square of the velocity, four times the momentum or intensity 

 is necessary to double the time of duration ; or, in other words, the intensity 

 of the pencil of rays is as the square of the time of the duration of that im- 

 pression made on the nerves of the retina. 



The new photometer consists of a brass bar fixed vertically in a stand, 

 carrying at its upper end a small tube in two parts, which may be lengthened 

 from 5 to 10 inches if requisite. This eye tube has at each end a sliding 

 plate pierced with holes of corresponding diameters. From the bottom of 

 the bar a projecting arm sustains the lower end of a strip of rolled steel 18 

 inches long, -n^ths inch broad, and ^nd inch thick ; this has at the upper 

 end a thin plate pierced N\ith a small liole, corresponding with the holes in 

 the sliders, and standing -ith of an inch from one of them : upon the main 

 bar is a prism with a slit in it, through which the strip of steel passes ; this 

 prism can be moved up or down by a rack and pinion, so as to lengthen or 

 shorten the vibrations of the strip. 



The method of using the instrument is to adjust the two holes at the oppo- 

 site ends of the horizontal eye tube, so that they perfectly correspond, and 

 do not permit any rays of light to enter, unless the plate at the extremity of 

 the spring be pushed aside. The light to be compared is tlien placed at a 

 certain given distance behind the plate, so that by bringing the axis of the 

 hole which is ]iierced in it into the axis of the tube, a small pencil of light 

 may enter the pupil of the eye. The prism is then placed at 100 of the scale 

 on the side of the brass bar, and the steel strip caused to vibrate gently. A 

 luminous disc immediately appears, accompanied by scintillations, which are 

 caused by the impressions on the retina being interrupted by dark intervals : 

 the prism is then gradually raised until the length of the vibrations of the 

 Strip being diminished, and the velocity increased, tlie luminous disc appears 

 perfectly steady and clear. The length of the vibrating portion of the strip 

 is then read off by the verniers marked on the brass rod, and compared with 

 the whole length of the spring, measured Irom 100, which is considered as 

 unity. The number of the vibrations to be computed from the found length 

 of the spring, are inversely to the numbers of vibrations of the whole length, 

 as the squares of their relative lengths. Hence are constructed the formula; 

 for calculation, which are given at length in the communication. 



A fresh luminous impression is made on the retina as often as the circular 

 aperture in the screen on the top of the spring cuts the axis of the tube. If 

 the duration of the small vibration of the nerves of the retina is shorter than 

 the time of a vibration of the spring, a dark interval appears between the two 

 luminons impressions. In this case the viluation of the spring is shortened 

 until the next impression returns just as the first ceases, and therefore the 

 dark interval disappears ; then by measuring the length of the shortened 

 spring, the number of vibrations can be computed, and from them the inten- 

 sity of the light. 



This communication was illustrated by a series of experiments npon diffe- 

 rent lights, with the Photometer which was presented by the author to the 

 Institution. 



" On the circumstances under which the Explosions of Steam Boilers gene- 

 rally occur, and on the means of preventing them." By Dr. Schafhaeutl, of 

 Munich, .\ssoc. Inst. C. E. 



Ex'jilosions of Steam Boilers. — In this communication it is assumed, that 

 perhaps not one-tenth of the recorded explosions of steam boilers can be 

 correctly attributed to the overloading of the safety valve, or to the accumu- 

 lation of too great a quantity of steam in the boiler. The author alludes to 

 the degree of pressure which hollow vessels, even of glass, are capable of 

 sustaining, if the pressure be applied gradually. He found, in repeating the 

 experiments of Cagniard de la Tour, subjecting glass tubes of one or two 

 inches in length, one-fourth part filled with water, hermetically sealed, and 

 immersed in a bath of melted zinc, that they apparently sustained the im- 

 mense pressure of 400 atmospheres without bursting; but if the end of an 

 iron rod was slightly pressed against the extremity of the tube, and the rod 

 caused to vibrate longitudinally by rubbing it with a leather glove covered 

 with resin, the tube was invariably shattered to pieces. 



Hence he concludes, that something more than the simple excess of pres- 

 sure of steam in the boiler is necessary to cause an explosion, and that a 

 slight vibratory motion alone, communicated suddenly, or at intervals, to 

 the boiler itself, might cause an explosion. From the circumstance of safety 

 valves having been generally found inefficient, he concludes that a force has 

 operated at the instant it was generated in tearing the bottom or sides of the 

 boiler, before it could act upon the safety valve. 



From the sudden effect of this force, explosions have been ascribed to the 



presence of hydrogen, generated by the decomposition of water ; but inde- 

 pendently of the difficulty of generating a large quantity of hydrogen in such 

 a manner, it could neither burn nor explode without the presence of a cer- 

 tain quantity of free oxygen or atmospheric air; and such an explosive mix- 

 ture would not take fire, even if mixed with 0-" of its own volume of steam.* 



Sudden conversion of Water into Sleam. — The ordinary mode of converting 

 water into steam is by successively adding small portions of <aloric to a rela- 

 tively large body of liquid ; but if the operation was reversed, and all the 

 heat imparted to a given (]uantity of water in one unit of time, an explosive 

 force would be developed at the same moment. For cxamjile, if a bar of 

 iron be heated until it is coated with liquid slag, and is then laid upon a 

 globule of water upon an anvil, and struck with a hammer, the liquid slag 

 communicates its caloric instantly to the water, becoming solid at the same 

 time that the water is converted into vapour with a loud report. A similar 

 occurrence may take place in a steam Ijoiler when a quantity of water is 

 thrown into contact with an overheated plate, either by a motion of the ves- 

 sel or from a portion of tlie incrustation formed on the bottom or sides be- 

 coming loosened. A sudden opening of the safety valve may, under certain 

 circumstances, prove dangerous, or even any rapid increase of heat which 

 would cause a violent excess of ebullition in the water. 



An examination is then entered into of the respective powers of water and 

 of steam, to transmit undulatory motion, and of their compressibility. Ac- 

 cording to Laplace, the conducting power of steam at our atmosphere and 

 ?041° Far. is 1041-3-1511 feet per second, and that of water 6036-88 feet. 

 The ratio of these different velocities is therefore as 1 : 4-5. 



In cases of a sudden explosive development of steam, the principal action is 

 directed against the bottom or the sides of the boiler, whence, spreading itself 

 through the water, it is finally transmitted through the steam to tiie safety 

 valve; a wave created by an explosion, even at the surface of the water, would 

 reach the bottom or the sides of the boiler, 4 1 times sooner than it would affect 

 the top of the steam chamber ; but if it took place at the bottom, the time 

 for the explosive wave to reach the safety valve would be the sura instead of 

 the difference of both velocities. Although these relative periods of time 

 may be considered as infinitely small, it is contended that there is sufficient 

 delay (counting from the moment at which the plates begin to yield) to cause 

 the rupture of the material which would otherwise have yielded by its own 

 elasticity had the time been greater, as all communication of motion is de- 

 pendant only on time. 



E-rperiments npon JJlres. — To illustrate the effect of the sndden develop- 

 ment of an explosive force upon the plates of a boiler, the author gives the 

 results of a series of experiments made by liim upon iron w-ires, for the pur- 

 pose of ascertaining the arammt of elongation which took place before yield- 

 ing under the sudden application of a given weight. The result was, that a 

 wire which had resisted a tension of 22 cwt. when gradually applied, broke 

 invariably, without any elongation, when the same force was suddenly applied 

 by a falling body. 



f.};on Railway Bars rf different qvalilies.. — Similar experiments with rail- 

 way bars showed that fibrous iron, which supported a gradual tension, broke 

 by the sudden application of the same force ; while close-grained iron, which 

 was incapable of resisting the gradual strain, bore perfectly well that of sud- 

 den impact. These facts are worthy of consideration in the selection of iron 

 for boiler plates, where the sudden action of the rending force is to be 

 guarded against. 



The details are then given of a series of experiments, illustrating in an in- 

 genious model, by means of an explosive mixture of chlorate of potassa, the 

 effects of explosions at different heights within a boiler. 



Proposed Safety-valve. — A careful examination of the circumstances, and 

 the results of his experiments, convinced the author that a simple mechanical 

 arrangement, applicable to all boilers, might be introduced, so as to diminish 

 the danger ai-ising from the sudden development of an explosive force. He 

 proposes to connect with the bottom of the boiler, by means of a pipe, an 

 extra safety valve of a given area, loaded to five-sixths of the absolute cohe- 

 sive force of the boiler plate, la the event of a sudden development of 

 steam, the first shock would act upon the valve and open it, which would 

 have the effect of depriving the wave generated of its destructive force, and 

 at the same time diminish the violence of the second shock from the top of 

 the boiler, having permitted the escape of a portion of the water from the 

 boiler. 



The apparatus for conducting the experiments -was presented with the 

 communication. 



Steam Boiler explosions. — Mr. Parkes stated, that he had been occupied 

 for several years in collecting facts illustrative of tl:e phenomena of steam 

 boiler explosions. These disasters could not all be referred to one cause. A 

 boiler might be too weak to sustain the pressure within it, and a rupture 

 would be the necessary conseqiience. But though the simple elastic force of 

 the steam might thus occasionally account for the rending of a boiler, that 

 cause was insufficient to explain many well-known phenomena, such as the 

 projection of an entire boiler from its seat, the separation of a boiler into two 

 parts, the one remaining quiescent, the other being driven to a great distance, 

 &c. He was of opinion that a very sudden development of force could alone 

 have produced such effects. 



Dr. Schafhaeutl had ingeniously shown that an explosive force generated 

 under water would act upon the bottom of the boiler and burst it, before the 



* See the author's experiments, Mechanics' Mag., Vol. XXX. p. 144. 



