8 



DISCOVERY 



quantities of material used in tlie production of the 

 various smokes, their relative screening powers could 

 then be calculated. These measurements have 

 naturally been carried out in an enclosed space, and it 

 is a moot point as to how far the results obtained arc 

 valid for the open air. The cfTicicncy of a smoke 

 screen depends to a great e.Ntcnt on the conditions of 

 lighting, and on how the observer is situated with 

 respect to the source of light. One has only to think 

 of the case of a room lit by a window looking on to the 

 street and covered with a screen or curtain of meshed 

 material. In the day-time a person in the room is 

 screened from obser\'ation from the outside, although 

 he can observe evcrytliing going on in the street ; at 

 night-time, when the room is lit up by an inside light, 

 the conditions are reversed. 



The fuel smoke which is usually made by vessels 

 of the fleet has already been mentioned. It has the 

 characteristic of most black smokes — namely, that it 

 is made of particles of unburnt or partly burnt car- 

 bonaceous matter, and is consequently far from hea\'y'. 

 1 1 is dissipated fairly rapidly, and only forms an effective 

 screen when continuously produced in large quantitj'. 



Practically all the effective moke screens which were 

 produced artificiallj' were w ite, and mention will now 

 be made of some of these. 



The first introduction of a smoke-producing agent 

 into the services involved the use of phosphorus, and 

 special apparatus was designed by Sir Richard Threlfall 

 for burning it at sea. The apparatus could be placed 

 on deck and the phosphorus fired in position, or it 

 could be modified in such a way that it floated when in 

 the sea and could be left functioning while the ship 

 proceeded on its course. There is no doubt that, 

 weight for weight, phosphorus has a great advantage 

 over practically all other smoke-producers, since only 

 oxygen is necessary for its combustion, and this is 

 available in the air in unlimited quantities. The 

 particles of phosphorus pento.xide produced are exceed- 

 ingly hygroscopic, and the cloud formed is a very good 

 one, although, because of the heavy nature of the par- 

 ticles, it tends to settle somewhat rapidl}'. Owing 

 to the great rise in temperature caused by the combus- 

 tion of the phosphorus, there is a distinct tendency 

 for the smoke to arch in low winds. This method of 

 producing smoke was used with great success early in 

 1917 for the purpose of screening monitors when 

 bombarding the Belgian coast. It was abandoned 

 later, however, partly owing to the dangerous nature 

 of phosphorus itself, when used on board ship, but 

 chiefly because of the extremely bright light which is 

 produced when phosphorus burns. The emission of 

 this light makes it an easy matter for guns to be ranged 

 on the vessel producing the smoke, and renders the 

 method quite useless for night operations. 



Since the supply of phosphorus was limited, and it was 

 urgently needed by the army as a smoke-producing 

 substance, the necessity arose of replacing it by some 

 smoke-producing material which could be stored in 

 appropriate floats on the decks of merchantmen or 

 other vessels, the floats then being lit and thrown 

 overboard when occasion demanded. The late Wing- 

 Commander Brock, K.N'., Commanding Oflicer of 

 the Royal Naval Experimental Station, Stratford, E., 

 devised a mixture which was really a firework com- 

 position, modified in such a way, by the addition of 

 resinous and other carbonaceous material, that a dense 

 smoke was produced on burning. This mLxture was 

 placed in tin canisters contained in a wooden box, 

 each canister being connected by a fuse to a match 

 composition (suitably protected) on the outside of 

 the box, and which could be fired by friction. Near 

 the top of the sides of the box were a number of small 

 holes, that were plugged by a composition which 

 would easOy be blown out by the pressure set up in 

 the box by the gases produced in the burning of the 

 smoke mixture. The boxes were so arranged that 

 they floated in water, the top side uppermost, and all 

 that was necessarj", in using them, was to strike the 

 friction tab and then, when it was certain that they had 

 started functioning correctly, throw them overboard. 

 The results obtained were not very satisf actor}', 

 however, owing to the difficulty of keeping the boxes 

 watertight, and the smoke-mixture free from moisture. 

 Also, the smoke produced suffered from the same de- 

 fects as the fuel-funnel-smoke. 



There are several chlorides of metals and non- 

 metals, such as stannic chloride, antimony penta- 

 chloride, silicon tetrachloride, etc., which give a good 

 smoke when decomposed by the moisture of the atmo- 

 sphere, or especially when a fine spray of the liquid is 

 mixed with a fine spray of Uquid ammonia ; stannic 

 chloride especialh' gives a very good cloud under such 

 conditions (see English Patent 127289). To use these 

 substances, however, they have to be stored in heavy 

 steel cylinders under a considerable pressure of air, 

 in order to force them out as a spray through a syphon 

 tube ; the dead weight of the containing-vessels is 

 thus very large in comparison with the weight of smoke- 

 producing material. Moreover, the contents of a 

 cylinder are discharged in the short time of 6-8 minutes. 

 This method of producing smoke is tliercfore not very 

 convenient on a large scale, but it has the advantage 

 that smoke can be produced instantly, and in quantitj', 

 by the mere turning of a tap. 



A smoke mixture which has found extended use, and 

 which depends on a principle not previously used, is 

 that devised by Berger. 



The English Patent 127031, which has been taken 

 out by Berger, is somewhat as follows : " Fumes or 



