ITS 



KNOWLEDGE 



[August 1, 189C. 



TT 



of smallest snrfaco, and therefore tbe proper equilibrium 

 figure of a liquid. In practice one seldom sees a perfectly 

 spLcrical drop, because gravitation towards the earth tends 

 to pull it out of shape, the more so the larger the drop. 

 Thus a largo hanging drop has the shape shown in Fig. 5 



— very much that of an india- 

 rubber bag filled with water. 



If one gently taps a bough 



on which the dewdrops hang, 

 Fig. 5.— a Pendcut Drop. each one trembles, the larger 



ones more slowly than the 

 little ones, for the total shrinking force of the elastic skin 

 is proportional to the area of the surface ; but the attraction 

 of the earth depends upon the mass of the drop, which is 

 proportional to the volume. The quivering of a soap-film 

 is more quickly brought to rest owing to the deadening 

 action of the air, which presses upon it from both sides. 



The elastic skin which capillarity provides accounts for 

 the regularly repeating form of a water jet which has 

 received some slight lateral displacement when issuing 

 from a nozzle. As Lord Eayloigh points out in describing 

 his beautiful researches upon liquid jets, the recurrent 

 form of the jet is due to vibrations of the fluid column 

 about the figure of equilibrium, superimposed upon a 

 general progressive motion. Since the phase of vibration 

 depends upon the time elapsed it is always the same at 

 the same point in space. The distance between consecutive 

 corresponding points of the recurrent figure (thr u-arc-leii(jth) 

 is proportional to the velocity. In fact, tbe recurrent 

 broadening and narrowing of a jet in which at the outset 

 some slight lateral disturbance has occurred, is comparable 

 to the series of stationary crests and hollows to leeward of 

 an obstacle in a shallow stream, but caused by capillarity, 

 not by gravity. When a water jet breaks up into drops, 

 owing to capillarity the drops strike against one another ; 

 and such is the elasticity of the skin of a water drop that, 

 instead of amalgamating at collision, the drops Hy apart 

 almost as if they were billiard balls, and to this is due the 

 scattering of the jet. Anything which helps the drops to 

 cohere at collision diminishes the scattering. This is the 

 effect produced by electrifying the spray, when the drops, 

 cohering, fall in a steady, rattling shower — like the rain in 

 a thunderstorm, says Professor Boys, whose " happy 

 thoughts" flow as readily as those of Mr. Burnand himself. 

 The pressure of wind and eddy upon either side of the 

 higher billows in a rising sea causes the tops of the ridges 

 to burst into spray. The least additional resistance on the 

 lee side makes these forced waves break more violently, the 

 masses of broken water which are flung bodily forward being 

 dangerous to vessels, especially the smaller craft. This 

 danger may be, to a great extent, avoided by diminishing the 

 capillary shrinking of the sea's surface by the use of oil. 

 When a drop of oil is placed upon water there are three 

 surfaces — the water-air surface, the oil-air surface, and the 

 oil-water surface. To these three surfaces there correspond 

 three stretching forces. If no two of these be greater than 

 the third the oil remains in a lens-shaped drop, as is the 

 case with the oily drops (which should not occur) upon hot 

 soup, hot water having a much smaller surface tension 

 than cold. In the case of cold water the tension of the 

 water-air surface is groater than the sum of the tensions of 

 the other two surfaces, and the oil cannot remain in a 

 thick drop, but is dragged out into an inconceivably thin 

 film, and spreads with surprising rapidity over a great 

 space of water. So much of the -jea as is covered with oil 

 has no longer a powerfully capillary surface, but a feebly 

 contractile oil surface, and there is no longer the same 

 liability of breaking and spraying. The waves are as high 

 as ever, but smoothly crested and comparatively harmless. 



Not much oil is required,* for there is no object in having 

 a tliick film ; half a gallon an hour is said to be suflicient 

 for a largo ship. It is best applied from a canvas bag 

 punctured in one or two places and hung over the side. 

 When lying-to in a heavy sea the bag should be hung jut 

 to windward. When drifting under shortened sail the oil 

 may be fed to leeward, so that tbe ship drifts into quiet 

 water. When a vessel is in tow, oil fed over the stern of 

 the tu" is very useful in preventing the sea from breaking 

 against the convoy. 



The surface tension and the density of water being 

 known, the force required to wrinkle the surface can be 

 calculated. The pressure of the air being known, the 

 velocity can be calculated which must be given to the 

 lower layers of the air in order that their motion should 

 wrinkle the surface, nothing like friction (/.<•., the mutual 

 engagement of rough surfaces) being supposed to occur. 

 This requisite wind velocity is calculated by Lord Kelvin 

 to be 12-8 knots, which, of course, is very much greater 

 than that which is found by observation to suffice for 

 rippling the surface of the sea. It appears, therefore, that 

 a part of the work of raising waves is due to something of 

 the nature of friction. This fluid friction is supposed to 

 be due to diftusiou. When air and water are in contact, 

 air is always dissolving in water and water is always 

 evaporating into air. In a closed vessel the water soon 

 saturates the air-space with moisture, and the water soon 

 absorbs as much air as it can hold. This does not mean, 

 however, that nothing goes on afterwards ; there is a 

 continual interchange, but evaporation and absorption 

 proceeding at equal rates no further change is observable. 

 So it is with the air and the sea : molecules of the gases of 

 air are continually dashing into the water and remain 

 there — for a time at least — and molecules of water are 

 continually flying up into the air. When the air is 

 blowing strongly over the surface of the water each 

 particle of air as it dashes into the sea helps to impel the 

 water forward ; the driving wind not only presses forcibly 

 upon the water but drags at it almost as a harrow, digging 

 with its teeth, scratches the soil along. 



AN EXPEDITION TO DISCOVER HOW CORAL 

 ISLANDS GROW. 



By E. W. Richardson. 



FIFTEEN years ago Darwin, finding surface investi- 

 gation and dredging insufficient to determine with 

 certainty the origin and genesis of coral atolls, 

 expressed in a letter to Alexander Agassiz the 

 wish that some rich man would have borings 

 made in some of the Pacific and Indian atolls, and bring 

 home cores from a depth of five hundred or six hundred 

 feet for examination. For nine years this idea lay dormant 

 in the minds of scientific men, but six years ago it took 

 shape, and a committee of leading geologists and biologists 

 was formed by the British Association to carry it out. 

 Professor Bonney was appointed chairman and Professor 

 Sollas secretary to this committee. The British Asso- 

 ciation appealed to the Royal Society, which readily 

 supported the scheme. A large sum was voted from the 

 Government Grant Committee, and another by the Royal 

 Society from its own funds. Professor Anderson Stuart, 

 of Sydney, N.S.W., has given great help, and it was 

 through his efforts that the Colonial Government was 



* A shoal of sprats makes a smooth patch of oily water which tells 

 the fishermen of their whereabouts. 



