DISCOVERY 



remain to make the climax of a great poem written 

 by Vergil, and written partly in aid of one of the 

 Emperor's great patriotic schemes. It is no longer 

 strange, but perfectly natural, that, in piecing together 

 three youthful poems {ArislcBus, Proteus, Orpheus) to 

 take the place of the lost Praises of Egypt and o/Galliis, 

 ingenious as the links inserted between them are, the 

 great poet should have had no heart in his work, and 

 should have left in despair the many broken threads of 

 the supposed story to be a silent record of the bitterest 

 sorrow of his life. 



Note. — Some account of the Greek and Latin inscriptions 

 was given in the Athencettm, March 14 and 21, 1896. Professor 

 Sayce discussed the Egj^ptian text in the Academy shortly 

 afterwards. Mommsen wTote an article in German in 

 Cosinopolis (London, 1896, p. 545), in which he is characteris- 

 tically silent about the statement of Ser^aus. The best book 

 for students to consult is Aus Vergih Friihzeit (pp. 140 ff., 

 Leipzic, 190 1), by the late Professor F. Skutsch. of Breslau, 

 whose work in this and in many ways opened a new epoch in 

 the study of Vergil. Some account of Callus and his relations 

 with Vergil will be found in a recent lecture of my own caUed 

 The Youth of Vergil (Manchester, 191 7). 



Smoke Screens at Sea 



By T. Slater Price, O.B.E., D.Sc. 



Lieul.-Commander R S' .V li. ; Head 0/ Chemistrg Depl., Birmingham 

 Municipal Teclmical School. 



In land warfare smoke screens have been used from 

 the earliest times for the purpose of hiding operations 

 in the field, but it was only during the recent war that 

 serious attention was given to the subject and attempts 

 made to put it on a proper scientific basis. In the 

 nav}', the method of producing a thick black smoke, 

 which can be emitted in great volume from the funnels, 

 has long been known, and tactical use made of it when 

 occasion demanded. During the war, however, as 

 the necessity arose for counteracting the submarine 

 warfare of the enemy against defenceless merchantmen, 

 and of undertaking operations against land stations 

 defended by modem resources, new methods of pro- 

 ducing smoke had to be devised. These methods had, 

 in many cases, to be independent of any mechanical 

 arrangements or motive power on board ship, since 

 when the vessel was being chased by a submarine, for 

 example, it was an advantage for it to be able to throw 

 overboard a float containing the smoke-producing 

 mixture, and thus hide its own course in the screen 

 produced. 



It is obvious that one essential condition to which 

 a smoke screen must conform is that of remaining near 



to the surface of the water or land. This condition is 

 perhaps more easily satisfied on sea than on land, since 

 on the latter, during the time when smoke screens are 

 of use — that is, in the day-time — the tendency is for the 

 ground to be hotter than the air above it ; the result 

 is that hot air is rising upwards continuaUy, and these 

 upward currents tend to make any smoke screen rise. 

 This effect is particularly noticeable on a bright, 

 sunshiny day. On the other hand, over the sea the 

 temperature gradient in the day-time is generally such 

 that the air near the sea is colder than the superin- 

 cumbent air, and the sea itself is at a lower temperature 

 than the air in contact with it. A smoke cloud at sea 

 will therefore tend to keep near the surface much more 

 readily than is the case on land. 



It is easily seen that, in order best to satisfy the con- 

 dition under consideration, the particles of the cloud 

 should be fairly heavy ; they must not, however, be 

 too heavy, otherwise the cloud will sink too rapidly 

 into the sea and thus lose its effectiveness as a screen. 

 It is very advantageous if the particles are of a hygro- 

 scopic nature, since there is then a tendency for the 

 cloud to become thicker for some time after it has been 

 produced, and not dissipate so quickly. In accordance 

 with this, it has been found on land that a denser and 

 lower cloud is formed when the atmosphere is nearly 

 saturated with moisture than when it is fairly dry ; 

 the conditions over the sea are consequently more 

 advantageous in this respect than over land. 



The velocity of the wind has a marked effect on the 

 smoke ; clouds which would tend to rise if produced 

 in still air, or in a low breeze, will be kept well down 

 when there is a good breeze blowing. It must be 

 remembered, however, that a cloud dissipates more 

 quickly in a wind of high than in one of low velocity, 

 so that for the same screening effect a much greater 

 amount of smoke must be generated in a given time 

 in a high wind than in a low one. This action of the 

 wind in keeping down the smoke is of very great 

 importance. Usually a great amount of heat is 

 generated in the reactions which produce some of the 

 most satisfactory smokes ; this gi%'es rise to strong 

 currents of hot gases, which would carry the smoke 

 upwards were it not for the counter- action of the wind. 

 When smoke is actually produced on board a ship under 

 weigh there is the distinct advantage over land condi- 

 tions that the smoke is sucked down to the surface of 

 the water by the inrush of air created at the stem of 

 the ship. 



Before dealing with the various methods lor making 

 smoke, the question of measuring the relative opacities 

 of different smokes may be briefly referred to. 

 Measurements have been made, both in this country 

 and in America, by determining the distances at 

 which an electric lamp is just obscured ; knowing the 



