1878.] Prof. Tyndall! Experiments on Fog-Signals. 2£7 



edge of a card in close proximity to one of the corners of the fork. 

 An immediate augmentation of the sound of the fork is the con- 

 sequence. 



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 considerably 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 Wool- 

 wich on the 4th of June, 1875, the sound-producing powers of four 

 different kinds of powder were determined. In the order of size of 

 grain they bear the names respectively of Fine-grain (F.G.), Large- 

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

 The charge in each case amounted to 4-|- 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 pronouncing the pebble-powder the worst sound- 

 producer. These differences are entirely due to differences in the 

 rapidity of combustion. All who have witnessed the performance of 

 the 80-ton gun at Woolwich must have been surprised at the mild- 

 ness of its thunder. To avoid the strain resulting from quick com- 

 bustion, the powder employed is composed of lumps far larger than 

 those of the pebble-powder above referred to. In the long tube of the 

 gun these lumps of solid matter gradually resolve themselves into gas, 

 which on issuing from the muzzle imparts a kind of push to the air, 

 instead of the sharp shock necessary to form the condensation of an 

 intensely sonorous wave. 



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 might be 

 presumed, be able to slip from places of condensation to places of rare- 

 faction with a rapidity sufficient to forestall the formation of the wave. 

 On a priori grounds then, we are entitled to infer the effectiveness of 

 gun-cotton, while in a great number of comparative experiments, 

 stretching from 1874 to the present time, *this inference has been veri- 

 fied in the most conclusive manner. 



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. General Campbell, 

 General Younghusband, Colonel Fraser, Colonel Maitland, and other 

 officers, have taken an active personal part in the investigation, and in 



