March 15, 191 7] 



NATURE 



45 



tion ; (ii) that of an adiabatic universe in which there 

 is no exchange of heat by radiation or contact. 



Reverting to the formula : if the gravitation exerted 

 on m increased by the fraction i/fe of itself for a 

 rise of i° C. in temperature, then at temperature 

 0° C. a change of gravitational energ}' into work 

 would be accompanied, on Carnot's principle, by a 

 gain of thermal energy equal to 273/fe of its amount, 

 which could scarcely escape notice unless fe were large. 



The sign in Dr. Todd's relation (i) seems to require 

 change. An objection applies to his hypothesis (2) 

 that it leads him to a temperature term in the law 

 of force which has the same value at all distances. 

 , Cambridge, March 4. J. L. 



The interesting letter from Dr. G. W. Todd in 

 N.ATURE of March i opens new ground on the subject 

 of gravitation and temperature. But does not the ex- 

 pression found need some modifications? Thus, in 

 the general case with m at temperature b and M at 

 temperature 6, we might write for the force the 

 expression 



F = G^4''+A (M log e + /« log e). 

 r- 



But even when thus generalised, the formula still 



seems unsatisfactory. Has Dr. Todd considered how 



to deal with the following curious facts? — (1) The 



•temperature corrections vanish at unxi temf>erature ; 



(2) at the temperature of absolute zero the attractive 

 force becomes minus infinity, i.e. an infinite repulsion'. 



(3) the- temperature correction is given as indepen- 

 , .dent of r, the distance apart of the masses. Hence 



t temperature correction to force , 



force itself 

 Thus, for large values of r, the temperature cor- 

 rection to the force might exceed the Newtonian 

 value of the force itself. E. H. Barton. 



University College, Nottingham, 

 March 8. 



Floating Earths. 



Will any of your readers kindly help in elucidating 

 a passage of Strabo? It occurs in his Book XHI., i., 

 67. The Greek is fairly plain, so a translation will 

 suffice : — 



• It is said that at Pitane the bricks float on water, 

 which has occurred also in the case of an islet in 

 Tyrrhenia; for the earth is lighter than an equivalent 

 bulk of water, so that it rides on the surface. Posi- 

 donius also says that he saw in Iberia a certain 

 argillaceous earth, used for taking moulds of silver 

 work, from which bricks were made which floated." 



The site of Pitane is the modern Chandarli, or 

 ^andarli, a small harbour on the west coast of .Asia 

 Minor, about 30 miles north of Smyrna and 15 miles 

 south-west of Bergama (Pergamon). Tyrrhenia is, of 

 course, Tuscany. Iberia is, no doubt, in this case 

 Spain. The name is also used for a district in the 

 Caucasus practically identical with the modern Georgia; 

 but without qualification it should mean Spain ; and 

 Strabo elsewhere quotes Posidonius as an authority 

 on natural phenomena in that country. 



The two "earths " which occur to me as floating' 

 on water are pumice and meerschaum. Meerschaum 

 is. of course, found in Asia Minor; Eski Shehr is the 

 principal source. But that is a long way from the 

 west coast, and I can find no record of its occurrence 

 anywhere near Pitane. In anv case, it does not seem 

 likely that it should have been used for building 

 purposes; and the use of the definite article, "the" 

 bricks (ras irXivffovs), seems to imply that the sub- 

 stance in question was the ordinar}' building material 

 of the district. 



NO. 2472, VOL. 99] 



Pitane lay on the south side of the Kara-dagh, a 

 large mass of eruptive rocks, andesite overlying tuffs, 

 as I learn from Philippson, who has described the 

 formation [Reisen u. Forsch. im westl. Kleinasien, 

 Petermann's Mitth., suppl. 167 (1910), p. 95). It 

 would seem, then, that in the case of Pit'>ne the 

 material must have been tuff. Is it possible that a 

 tuff porous enough to float on water could be used 

 as a building material, or that passably durable 

 "bricks" could be made of it? 



The floating "islet" in Tuscany may, I suppose, 

 have been a mass of pumice from the Lipari Islands 

 drifting northwards. Such islets are recorded to have 

 floated about the ^gean Sea after the great eruption 

 of Santorin in 1650, and even to have blocked some 

 of the ports. But though the Lipari Islands contain 

 enormous masses of pumice light enough to float. I 

 can find no record of any having been actually ejected 

 into the sea in historical times. Can anyone tell me 

 if there is such a record? 



The "argillaceous earth " in Spain clearly cannot 

 have been tuff or pumice. Can it have been meer- 

 schaum? This material is, of course, capable of 

 delicate carving; would it be suitable for making 

 moulds for copying silver work? It is said that there 

 are limited deposits of it in Spain. It may be noticed 

 that the quotation from Posidonius does not imply 

 that it was actually used for building, but says only 

 that bricks " formed " (mr/wttf'^s) from it will float. 

 This may mean no more than that blocks of the shape 

 and size of a brick will float. I am not clear as to 

 the exact meaning of irrjywfifvas — whether it implies 

 anv process more than mere cutting. One would 

 naturally expect it to mean "congealed " or "solidi- 

 fied." Meerschaum is, I understand, soft when dug, 

 but hardens on exposure to the air. Perhaps, there- 

 fore, the word may mean "hardened." On the other 

 hand, I feel confident that in the ohrase »/ ra dpyvpatfiara 

 tKiiOTTfTai the verb is used in the technical sense of 

 taking a mould or impression, not, as some have 

 thought, with the trivial meaning, " with which silver 

 work is cleaned." 



Anyone who can enlighten an ignorant Hellenist 

 on these points would be assured of his gratitude- 



Walter Le.\f. 



6 Sussex Place, Regent's Park, N.W., February' 24. 



SCIENTIFIC ASPECTS OF FUEL 

 ECONOMY.^ 



THE appointment of a Board of Fuel Research 

 by the Committee of the Privy Council for 

 Scientific and Industrial Research on the recom- 

 mendation of the -\dvisory Council was noted in 

 N.ATURE of February 22, and something' may 

 perhaps be said with advantage on the real and 

 pressing need for securing, so far as it may be 

 done by oflBcial efforts, the fullest investigation 

 of the potentialities of fuel. 



To those occupied with the study of the 

 scientific utilisation of coal, the crude and wasteful 

 way in which we, as a nation, have been mal- 

 treating our supplies of that irreplaceable raw 

 material has been for some time past a depressing 

 spectacle. Recognition of the value of purely 

 thermal economies has been fairly general, 

 although the quantities of fuel used in feeding 



1 Report of the British .Association Cinunittce on Fuel Economy, 

 September, 1416. 



Memorandum by Chief Engineer, the Manchester Steam Users' .Associa- 

 ion, for the year 1913. 



