272 



NA TURE 



[March 2, 1922 



of force, or in any case into the vertical plane where 

 the curvature of the equipotential surfaces is a mini- 

 mum. The turning moment of the direct effect of the 

 earth's field about a vertical axis is precisely the 

 same for the floating body as for a like body when 

 suspended. For the total effect of the floating body 

 we must consider also the turning moment of the 

 fluid pressure, or of the equivalent body force, and 

 this, in general, opposes the direct turning effect of 

 the earth's field. 



For a body like an elongated right cylinder the two 

 turning effects practically cancel each other, so that 

 the resultant is an infinitesimal of higher order, so 

 to speak, than either turning effect by itself. It is 

 possible, however, by varying the shape of the 

 elongated body to make one or the other tendency 

 prevail. If the body overhangs the fluid considerably, 

 like the bow of a racing yacht, the direct effect of 

 the earth's field has the advantage of position in 

 producing a turning moment, and in the normal case 

 the tendency of the body is to turn into the prime 

 vertical, just as for the suspended body. If the body 

 has its extreme end submerged, thus resembling the 

 mirror images of the ends of the overhanging body, 

 then the contrary tendency will prevail, and in the 

 normal case the body will tend to set itself in the 

 meridian. This tendency to seek the meridian would 

 not, however, be true of all elongated floating bodies 

 of dimensions comparable with those of the Eotvos 

 balance, as Col. Grove-Hills would seem to imply. 



My own interest in the effect of the earth's gravity 

 field on floating bodies was due originally to an 

 attempt to account for certain hypothetical displace- 

 ments of each continental mass as a whole towards 

 the equator. These displacements are believed by a 

 well-known geologist — who for the present, however, 

 does not wish to be quoted by name — to be estab- 

 lished almost beyond question. His ideas differ 

 somewhat from those of Prof. Alfred Wegener, of 

 Marburg, who has published much regarding sup- 

 posed continental displacements. The problem of the 

 equilibriurn of the mass of self-attracting gravitating 

 fluid rotating about an axis, with a mass of lighter 

 matter floating in the fluid and projecting out of it, 

 is apparently one of considerable difficulty, especially 

 if we consider the gravitational effects of the floating 

 body on the field of force. Considerations of sym- 

 metry would lead us to suppose, however, that the 

 floating body would not be in stable equilibrium at 

 any random point on the surface of the body ; the 

 equator of the rotating fluid seems a natural place 

 for stability, and a calculation of the forces acting 

 shows that there is, in fact, a tendency for a floating 

 body to move towards the equator — a tendency 

 stronger, in general, the higher the body floats above 

 the free surface of the fluid. 



The difficulty with this equatorward tendency as an 

 explanation of the supposed movements of the con- 

 tinental masses is that the movements appear to have 

 occurred after the earth's crust was well consoUdated 

 and there could be no longer any question of floating 

 continental blocks. There is, to be sure, a region of 

 weaker and softer crust around the edge of each con- 

 tinental block, where a sort of syncUne dips into 

 the warmer regions nearer the centre of the earth. 

 (All this geology is at second hand, or worse, and 

 should be accepted only with appropriate reserva- 

 tions.) There would be also the weakness under- 

 neath the continental mass due to the heat there. 

 In a way this condition resembles that of a floating 

 body, and if for any reason the continental mass 

 should move, the region of weakness around its edges 

 would move with it. However, it does not seem 

 especially probable that the weak gravitational field 

 that tends to move a floating body towards the 



NO. 2731, VOL. 109] 



equator could accomplish very much in moving a 

 continent that forms part of a fairly well consoli- 

 dated crust. 



Even though the equatorward force on a floating 

 body may not be manifest in the displacement of 

 continents, it may perhaps be discernible in the 

 motions of much smaller floating bodies, namely, 

 icebergs. The higher the iceberg the stronger this 

 force. The acceleration may be written approxi- 

 mately for the normal case as 



-A^ sm 2^, 



where d is the distance between the centre of gravity 

 of the floating body and its centre of buoyancy, 

 A^ the difference between the acceleration of gravity 

 at pole and equator, ^ the latitude, and a the radius 

 of the earth. An iceberg 200 metres in height is 

 rather exceptional for Arctic latitudes, but in 

 Antarctic waters a height of 500 metres (1700 ft.) 

 has been reported (see " The Seaman's Handbook of 

 Meteorology," published by H.M. Stationery Ofi&ce 

 for the Meteorological Committee, third edition, pp. 

 132-35). If we suppose the icebergs to be plateaux 

 with wall-sides and to have only one-eighth of their 

 masses above the water, the values of d corresponding 

 to visible heights of 200 and 500 metres would be 

 100 metres and 250 metres respectively. 



The value of A^ is 5-18 cm. and of a 6-37 x 10* 

 cm. The maximum value of the equatorward ac- 

 celeration on the two icebergs, which occurs in latitude 

 45°, would be o-oooo8i and 0-000203 cm. per sec. 

 per sec. respectively. At latitude 60°, the latitude 

 of Cape Farewell in Greenland, these figures would be 

 reduced to 0-000070 and 0-000176 cm. respectively ; 

 but even the smallest of the four accelerations acting 

 for an entire day would, if unresisted, set the iceberg 

 in motion, give it a velocity of more than 6 cm. per 

 sec. at the end of the day, and move it 2-6 km. At 

 the end of twenty days the velocity would be 1-2 

 metres per sec. and the displacement 1050 km., or 

 more than nine degrees of latitude. With greater 

 acceleration the effects would be greater in proportion . 



It is fairly certain, however, that the resistance of 

 the water would prevent the iceberg from actually 

 attaining any of the larger velocities. Probably the 

 terminal velocities from these small forces are of the 

 order of magnitude of a very few centimetres per 

 second. The dominant forces are the winds and 

 currents, but these small forces arising from the 

 earth's field would act more effectively on the higher 

 icebergs and bring them more rapidly into low 

 latitudes. One gets the impression in reading 

 accounts of ice observed in low latitudes that large 

 icebergs are the rule there rather than the exception. 

 There are some obvious reasons for this. The large 

 icebergs are less apt to be overlooked and better able 

 to survive the warm weather than are the small ones. 

 The selective effect of the earth's field is merely an 

 additional reason for the frequent occurrence of large 

 icebergs in low latitudes ; to say how important a 

 reason it is would seem to require more data than 

 we now have. Walter D. Lambert. 



U.S. Coast and Geodetic Survey, Washington, 

 D.C., January 20. 



Revival of Sporophores of Schizophyllum 

 commune, Fr. 



• As has been pointed out by Prof. A. H. R. Buffer 

 ("Researches on Fungi," 1909, p. 113), sporophores 

 of Schizophyllum commune which have curled up as 

 the result of definite xerotropic action can be revived 

 by suitable treatment in a moist chamber. The 

 following illustrations afford interesting photographic 

 confirmation of Prof. Buffer's experiments : — 



