252 Prof. Tyndall. Experiments on Fog- Signals. [Mar. 21, 



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 absolutely free from cloud. From the deck of 

 the " Galatea" a rocket was discharged, which reached a great eleva- 

 tion, and exploded with a loud report. Following this solid nucleus 

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

 tinually greater distance, dying gradually off into silence after seven 

 seconds' duration. These echoes were of the same character as those 

 so frequently noticed at the South Foreland in 1872-73, and called by 

 me " aerial echoes." 



On the 23rd of March the experiments were resumed, the most 

 noteworthy results of that day's observations being that the sounds 

 were heard at Tillingham, 10 miles to the 1ST.E. ; at West Mersea, 15f 

 miles to the KE. by E. ; at Brightlingsea, 17 J. miles to the KE. ; and 

 at Clacton Wash, 20^- miles to the KE. by \ E. The wind was blow- 

 ing 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 Maroon. 



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 1'03. 

 while that of the powder was T70. 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 instance of Mr. Mackie, who had been in communica- 

 tion previously with the Deputy Master of the Trinity House and 

 myself, a Committee of the Elder Brethren visited the cotton-powder 

 manufactory, on the banks of the Swale, near Faversham, on the 16th 

 of June, 1877. The weights of cotton-powder employed were 2 oz., 

 8 oz., 1 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 limited our distance to 1\ 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 the material exploded, 8 oz. yielding 

 very nearly as loud a report as 1 lb. The " aerial echoes," which in- 

 variably followed the explosion of the rockets, were loud and long- 

 continued, shading off, as in all previous cases, by imperceptible grada- 

 tions into silence. 



On the 17th of October, 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 



