April \, 1878] 



NATURE 



457 



observations made on board the Galatea at distances varying from 

 if mile to 4'8 miles, w^ere as follows : — 



Weights 

 Va'ue of sound 



4-0Z. 

 3-12 



6-oz. 9-0Z. 

 3 '34 40 



12-0Z. 

 4"03 



These charges were cut from a slab of dry gun-cotton about 

 1 1 inch thick ; they were squares and rectangles of the following 

 dimensions: — 40Z. , 2 Inches by 2 inches; 6 oz., 2 inches by 

 3 inches ; 9 oz., 3 inches by 3 inches; 12 oz., 2 inches by 6 

 mches. 



It is an obvious corollary from the foregoing experiments that 

 on our " nesses " and promontories, where the land is clasped on 

 both sides for a considerable distance by the sea, — where, there- 

 fore, the sound has to propagate itself rearward as well as for- 

 ward — the use of the parabolic gun, or of the parabolic reflector 

 might be a disadvantage rather than an advantage. Here gun- 

 cotton, exploded in the open, forms the most appropriate source 

 of sound. This remark is especially applicable to such light- 

 ships as are intended to spread the sound all round them as from 

 central foci. As a signal in rock lighthouses, where neither syren, 

 steam-whistle, nor gun could be mounted, and as a handy fleet- 

 signal, which dispenses with the lumber of special signal-gun', 

 the gun-cotton will prove invaluable. But in most of these cases 

 we have the drawback that local damage may be done by the 

 explosion. The lantern of the rock-lighthouse might suffer from 

 concussion near at hand, and though mechanical arrangements 

 might be devised, both in the case of the lighthouse and of the 

 ship, to place the firing-point of the gun-cotton at a safe 

 distance, no such arrangement could compete, as i^egards sim- 

 plicity and effectiveness, with the expedient of a gun-cotton 

 rocket. Had such a means of signalling existed at the Bishop's 

 Rock Lighthouse, the ill-fated Schiller might have been warned 

 of her approach to danger ten, or it may be twenty, miles before 

 she reached the rock which wrecked her. Had the fleet pos- 

 sessed such a signal, instead of the ubiquitous but ineffectual 

 steam-whistle, the Iron Duke and Vanguard need never have 

 come into collision. 



It was the necessity of providing a suitable signal for rock 

 lighthouses, and of clearing obstacles which cast an acoustic 

 shadow, that suggested the idea of the gun-cotton rocket to Sir 

 Richard Collinson, Deputy Master of the Trinity House. That 

 idea was to place a disk or short cylinder of the gun- cotton, 

 which had proved so effectual at low levels, in the head of a 

 rocket, the ascensional force of which should be employed to 

 carry the disk to an elevation of 1,000^ feet or thereabouts, 

 where by the ignition of a fuse associated with a detonator, the 

 gun-cotton should be fired, sending its sound in all directions 

 vertically and obliquely down upon earth and sea. The first 

 attempt to realise this idea was made on July 18, 1876, at the 

 firework manufactory of the Messrs. Brock, at Nunhead. Eight 

 rockets were then fired, four being charged with 5 oz. and four 

 with 7^ oz. of gun-cotton. They ascended to a great height, 

 and exploded with a very loud report in the air. On July 27, the 

 rockets were tried at Shoeburyness. The most noteworthy result 

 on this occasion was the hearing of the rockets at the Mouse 

 Lighthouse, 8^ miles E. by S., and at the Chapman Lij^hthouse, 

 Si miles \V. by N. ; that is to say, at opposite sides of the firing- 

 point. 



On December 13, 1876, and again on March 8, 1877, com- 

 parative experimenes of firing at high and low elevations were 

 executed. The gun-cotton near the ground consisted of \Vo. 

 disks suspended from a horizontal iron bar about 4^ feet above 

 the ground. The rockets carried the same quantity of gun- 

 cotton in their heads, and the height to which they attained, as 

 determined by a theodolite, was from 800 to 900 feet. The day 

 last-mentioned was cold, with occasional squalls of snow and 

 hail, the direction of the sound being at right angles to that of the 

 wind. Five series of observation were made on board the Vestal 

 at distances varying from three to six miles. The mean value 

 of the explosions in the air exceeded that of the explosions near 

 the ground by a small but sensible quantity. At Windmill Hill, 

 Gravesend, however, which was nearly to leeward, and 5J miles 

 from the firing-point, in nineteen cases out of twenty-lour the 

 disk fired near the ground was loudest ; while in the remaining 

 five the rocket had the advantage. 



Towards the close of the day the atmosphere became very 

 serene. A few distant cumuli sailed near the horizon, but 

 the zenith and a vast angular space all round it were abso- 

 lutely free from cloud. From the deck of the Galatea a 

 rocket was discharged, which reached a great elevation, and 

 exploded with a loud report, rollowinp this solid nucleus of 



sound was a continuous train of echoes, which retreated to a 

 continually greater distance, dying gradually off into silence after 

 seven seconds' duration. These echoes were of the same cha- 

 racter as those so frequently noticed at the South Foreland in 

 1872-73, and called by me "aerial echoes." 



On March 23, the experiments were resumed, the most note- 

 worthy results of this day's observations being that the sounds 

 were heard at Tillingham, 10 miles to the N. E. ; at West 

 Mersea, 15! miles to the N.E. by E.; at Brightlingsea, \']\ 

 miles to the N.E. ; and at Clacton Wash, 20^ miles to the 

 N.E. by \ E. The wind was blowing at the time from the S.E. 

 Some of these sounds were produced by rockets, some by a 

 24-lb. howitzer, and some by an 8 inch M aroon. 



In December, 1876, Mr. Gardiner, the managing director of 

 the Cotton-powder Company, had proposed a trial of this 

 material against the gun-cotton. The density of the cotton, he 

 urged, was only I 03, while that of the powder was 170. A 

 greater quantity of explosive material being thus compressed 

 into the same volume, Mr. Gardiner thought that a greater 

 sonorous effect must be produced by the powder. At the in- 

 stance of Mr. Mackie, who had previously gone very thoroughly 

 into the subject, a Committee of the Elder Brethren visited the 

 cotton powder manufactory, on the banks of the Swale, near 

 Faversham, on June 16, 1877. The weights of cotton powder 

 employed were 2 oz., 8 oz., i lb., and 2 lbs., in the form of 

 rockets and of signals fired a few feet above the ground. The 

 experiments throughout were arranged and conducted by Mr. 

 Mackie. Our desire on this occasion was to get as near to 

 windward as possible, but the Swale and other obstacles 

 lifted our distance to \\ mile. We stood here E. S.E. 

 from the firing-point while the wind blew fresh from the N. E. 

 The cotton-powder yielded a very effective report. The 

 rockets in general had a slight advantage over the same quantities 

 of material fired near the ground. The loudness of the sound 

 was by no means proportional to the quantity of th2 material 

 exploded, 8 oz. yielding very nearly as loud a report as I lb. 

 The "aerial echoes," which invariably followed the explosion 

 of the rockets, were loud and long-continued. 



On October 17, 1877, another series of experiments with 

 howitzers and rockets was carried out at Shoeburyness. The 

 charge of the howitzer was 3 lbs. of L.G. powder. The charges 

 of the rockets were 12 oz, 8 oz., 4 oz., and 2 oz, of gun-cotton 

 respectively. The gun and the four rockets constituted a series, 

 and eight series were fired during the afternoon of the 17th. 

 The observations were made from the Vestal and the Galatea, 

 positions being assumed which permitted the sound to reach the 

 ob.-ervers with the wind, against the wind, and across the wind. 

 The distance of the Galatea varied from three to seven miles, 

 that of the Vestal, which was more restricted in her movements, 

 being from two to three miles. Briefly summed up, the result is 

 that the howitzer, firing a 3-lb. charge, which it will be remem- 

 bered was our best gun at the South Foreland, was beaten by 

 the 12-0Z. rocket, by the 8-oz. rocket, and by the 4-oz. rocket. 

 The 2-oz. rocket alone fed behind the howitzer. 



On the following day, viz., October 18, we proceeded to 

 Dungeness with the view of making a series of strict comparative 

 experiments with gun-cotton and cotton- powder. Rockets con- 

 taining 8 oz., 4 oz., and 2 oz. of gun-cotton had been prepared 

 at the Ro} al Arsenal ; while others, containing a similar quantity 

 of cotton-powder, had been supplied by the Cotton-powder 

 Company at Faversham. With these were compared the ordinary 

 i8-pounder gun, which happened to be mounted at Dungeness, 

 firing the usual charge of 3 lbs. of powder, and a syren. 



From these experiments it appeared that the gun-cotton and 

 cotton-powder were practically equal as producers of sound. 



The effectiveness of small charges was illustrated in a very 

 striking manner, only a single unit separating the numerical 

 value of the 8-oz. rocket from that of the 2-oz. rocket. The 

 former was recorded as 6 "9 and the latter as 5 '9, the value of 

 the 4-0Z. charge being intermediate between them. These results 

 were recorded by a number of very practised observers on board 

 the Galatea. They were completely borne out by the observa- 

 tions of the Coastguard, who marked the value of the 8-oz. 

 rocket 6'i, and that of the 2-oz rocket 5*2. The i8-pounder 

 gun fell far behind all the rockets, a result probably to be 

 in part ascribed to the imperfection of the powder. The 

 perlormance of the syren was, on the whole, less satisfac- 

 tory than that of the rocket. The instrument was worked, not 

 by steam of 70 lbs. pressure, as at the South Foreland, but by 

 compressed air, beginning with 40 lbs. and ending with 30 lbs. 

 pressure. The trumpet was pointed to windward, and in the 



