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NATURE 



[April \, 1878 



compendium of chemical knowledge. Although Germany takes 

 the lead in regard to chemical discovery, she is far behindhand in 

 this respect. The newedition of the " Hand worterbuch," based on 

 the well-known work of Liebig, Wohler, and Kolber, now edited 

 by Prof. Fehling, was commenced in 1871, but has progressed 

 at a snail's pace, being only half way through the letter E, and 

 the second of the six volumes which it will compose, not being 

 yet completed. The Italian chemists have recently issued a 

 dictionary of chemistry on a somewhat smaller scale than those 

 alluded to above, but well edited and written. 



The additions to the Zoological Society's Gardens during the 

 past week include two Pudua Deer (Cervus humilis) from Chili, 

 a Black-faced Spider-Monkey {Attles aier) from East Peru, 

 deposited ; an Arabian Gazelle {Gazella arabica) from Arabia, 

 presented by Mr. W. W. Webb. 



FOG SIGNALS^ 



TTSURING the long, laborious, and, I venture to think, memor- 

 ^-^ able series of observations conducted under the auspices of 

 the Elder Brethren of the Trinity House at the South Foreland 

 in 1873 and 1873, it was proved that a short 5J inch howitzer, 

 firing 3 lbs. of powder, yielded a louder report than a long 

 l8-pounder firing the same charge. Here was a hint to be 

 acted on by the Elder Brethren. The effectiveness of the sound 

 depended on 'the shape of the gun, and as it could not be 

 assumed that in the howitzer we had hit accidentally upon the 

 best possible shape, arrangements were made with the War 

 Office for the construction of a gun specially calculated to pro- 

 duce the loudest sound attainable from the combustion of 3 lbs. 

 of powder. To prevent the unnecessary landward waste of the 

 sound, the gun was furnished with a parabolic muzzle, intended 

 to project the sound over the sea, where it was most needed. 

 The construction of this gun was based on a searching series of 

 experiments executed at Woolwich with small models, provided 

 with muzzles of various kinds. The gun was constructed on the 

 principle of the revolver, its various chambers being loaded and 

 iDrought in rapid succession into the firing position. The 

 performance of the gun proved the correctness of the principles 

 on which its construction was based. 



Coincident with these trials of guns at Woolwich gun-cotton 

 was thought of as a possibly effective sound-producer. From 

 the first, indeed, theoretic considerations caused me to fix my 

 attention persistently on this substance ; for the remarkable 

 experiments of Mr. Abel, whereby its rapidity of combustion and 

 violently explosive energy are demonstrated, seemed to single it 

 out as a substance eminently calculated to fulfil the conditions 

 necessary to the production of an intense wave of sound. What 

 those conditions are we shall now more particularly inquire, 

 calling to our aid a brief but very remarkable paper, published 

 by Prof. Stokes in the Philosophical Magazijie for 1868. 



A sound wave consists essentially of two parts — a condensa- 

 tion and a rarefaction. Now air is a very mobile fluid, and if 

 the shock imparted to it lack due promptness, the wave is not 

 produced. Consider the case of a common clock pendulum, 

 which oscillates to and fro, and which therefore might be ex- 

 pected to generate corresponding pulses in the air. When, for 

 example, the bob moves to the right, the air to the right of it 

 might be supposed to be condensed, while a partial vacuum 

 might be supposed to follow the bob. As a matter of fact, we 

 have nothing of this kind. The air particles in front of the bob 

 retreat so rapidly, and those behind it close so rapidly in, that 

 no sound-pulse is formed. 



The more rapid the shock imparted to the air, the greater is 

 the fractional part of the energy of the shock converted into 

 wave motion. And as different kinds of gunpowder vary con- 

 siderably in their rapidity of combustion, it may be expected 

 that they will also vary as producers of sound. This theoretic 

 inference is completely verified by experiment. In a series of 

 preliminary trials conducted at Woolwich on the 4th of June, 

 1875, the sound-producing powers of four different kinds of 

 powder were determined. In the order of their sizes they bear i 

 the names respectively of Fine-grain (F.G.), Large-grain (L.G.), I 



I "Recent Experiments on Fog Signals." Abstract of paper read at the ' 

 Royal Society, March 21. By Dr. XyEdall, F.R.S., Professor of Natural i 

 JPhilosophy in the Royal Institution. ( 



Rifle Large-grain (R.L.G.), and Pebble-powder (P.). The 

 charge in each case amounted to 4I lbs., four 24-pound 

 howitzers being employed to fire the respective charges. There 

 were eleven observers, all of whom, without a single dissentient, 

 pronounced the sound of the fine-grain powder loudest of all. 

 In the opinion of seven of the eleven the large-grain powder 

 came next ; seven also of the eleven placed the rifle large-grain 

 third on the list ; while they were again unanimous in pro- 

 nouncing the pebble-powder the worst sound-producer. These 

 differences are entirely due to differences in the rapidity of 

 combustion. 



These are some of the physical reasons why gun-cotton might 

 be regarded as a promising fog-signal. Firing it as we have been 

 taught to do by Mr. Abel, its explosion is more rapid than that 

 of gunpowder. In its case the air- particles, alert as they are, will 

 not, it may be presumed, be able to slip from places of con- 

 densation to places of rarefaction with a rapidity sufficient to 

 forestall the formation of the wave. 



As regards explosive material, and zealous and accomplished 

 help in the use of it, the resources of Woolwich Arsenal have 

 been freely placed at the disposal of the Elder Brethren. Gen. 

 Campbell, Gen. Younghusband, Col. Eraser, Col. Maitland, and 

 other officers, have taken an active personal part in the inves- 

 tigation, and in most cases have incurred the labour of reducing 

 and reporting on the observations. Guns'~of various forms and 

 sizes have been invoked for gunpowder, while gun-cotton has 

 been fired in free air, and in the foci of parabolic reflectors. 



On February 22, 1875, a number of small guns, cast specially 

 for the purpose — some with plain, some with conical, and some 

 with parabolic muzzles, firing 4 oz. of fine-grain powder, were 

 pitted against 4 oz. of gun-cotton, detonated both in the open 

 and in the focus of a parabolic reflector. The sound produced 

 by the gun-cotton, reinforced by the reflector, was unanimously 

 pronounced loudest of all. With equal unanimity, the gun- 

 cotton detonated in free air was placed second in intensity. 

 Though the same charge was used throughout, the guns differed 

 considerably among themselves, but none of them came up to 

 the gun-cotton either with or without the reflector. A second 

 series, observed from a different distance on the same day, con- 

 firmed to the letter the foregoing result. 



Meanwhile, the parabolic muzzle-gun, expressly intended 

 for fog-signalling, was pushed rapidly forward, and on March 

 22 and 23, 1876, its power was tested at Shoeburyness. Pit- 

 ted against it were a l6-pounder, a S^-inch howitzer, ijlb. of 

 gun-cotton detonated in the focus of a reflector, and ij lb. of 

 gun-cotton detonated in free air. Oa this occasion, nineteen 

 different series of experiments were made, when the new experi- 

 mental gun, firing a 3 -lb. charge, demonstrated its superiority 

 over all guns previous'y employed to fire the same charge. As 

 regards the comparative merits of the gun-cotton fired in the 

 open, and the gunpowder fired from the best constructed gun, 

 the mean values of their sounds were found to be the same. 

 Fired in the focus of the reflector, the gun-cotton clearly domi- 

 nated over all the other sound-producers. ^ 



The whole of the observations here referred to were em- 

 braced by an angle of about 70°, of [which 50° lay on the 

 one side and 20° on the other side of the line of fire. The shots 

 were heard by eleven observers on board the Galatea, which took 

 up positions varying from 2 miles to 13J miles from the firing- 

 point. In all these observations, the reinforcing power of the 

 reflector, and of the parabolic muzzle .of the gun, came into 

 play. But the reinforcement of the sound in one direction im- 

 plies its withdrawal from some other direction, and accordingly 

 we find that at a distance of $^ miles from the firing-point, and 

 on a line, including nearly an angle of 90°, with the line of fire, 

 the gun-cotton in the open beat the new gun ; while behind 

 the station, at distances of 8^ miles and 13I miles respectively, 

 the gun-cotton in the open beat both the gun and the gun-cotton 

 in the reflector. This result is rendered more important by the 

 fact that the sound reached the Mucking Light, a distance of 

 132 miles, against a light wind which was blowing at the time. 



Theoretic considerations render it probable that the shape of 

 the exploding mass would affect the constitution of the wave of 

 sound. I did not think large rectangular slabs the most favour- 

 able shape, and accordingly proposed cutting [3. large slab into 

 fragments of different sizes, and pitting them against each other. 

 The differences between the sounds were by no means so great 

 as the differences in the , quantities of explosive material might 

 lead one to expect. The mean values of eighteen series of 



^ In this case the reflector was fractured by the explosion. 



