November 8, 1894] 



NA TURE 



The Planting of Timber Trees. 



In Traill's sketch of the life of Shaftesbury (the first Earl), 

 the following pissage occurs in a letter from ttte Earl to the 

 steward of his estates in Dorsetshire ; — 



" The best planting of timber trees is with nuts, acorns, seeds, 

 and footsets, and not with young trees removed . . . Insetting 

 of chesnuts, ac jrns, and seeds [it is desirable] to steep them 

 twenty-four hours in milk, which gives them a great advantage. 

 . . . If siccamotei [are planted] near my gardens, they will spoil 

 all my fruit with the llies they breed. Therefore pray pluck up 

 all the siccamores that are in the dry meadow behind my 

 kitchen-garden, an 1 in the room of every one of them plant a 

 che.snut, a walnut, or a honey-broke oak." 



Can any of yoar readers inform me whether the soaking of 

 seeds in milk is now, or ever has been, ex'ensively practised, also 

 what is meant by a " honey-broke oak" ? 



Alfred W. Be.\nett. 



Rhynchodemus Terrestris in England. 



The credit of the first discovery of this Land-planarian in 

 England lies not with Sir John Lubbock, as Dr. ScharfT stated, 

 but with the late Rev. L. Jfnyns (Blomefield), who, in his 

 "Observations in Natural History," 1S46 (p. 315), makes some 

 interesting remarks on the " Ground Fluke " [Fasciohi terrestris) 

 and its occurrence in the woods at Bottisham Hall, a locality 

 searched with success by .Mr. Harmer. 



Rhynchodemus terrestris is widely distributed in England, 

 and I have found it in Derbyshire, North Lancashire, and 

 Westmoreland, under moist conditions and on a limestone 

 substratum. 



.•\ny additions to the limited number of land-planarians in 

 Europe are of considerable interest, and mention may therefore be 

 madeof Prof. v. Graff's description {Bu'.l.Soc.Zool, France, xviii. 

 189J, pp. 122-3), of Rhynchodemus fyrenaiciis, n. sp., from 

 St. Jean de Luz, which is not alluded to by Dr. Scharff. 



F. W. Gamki.e. 



Oivens College, Manchester, October 26. 



Tan-Spots over Dogs' Eyes. 



The shepherds in some of the east counties of Scotland used 

 to call their bhck-and-tan collies four-eyed dogs, which agrees 

 so far with Mr. Peal's observations. These collies, twenty 

 years ago, were much in demand. Now they are hardly 

 allowed prizes at show-, and are becoming scarce ; black and 

 white, pure white, and, more commonly, brown dogs being 

 greater favourites. J. Shaw. 



A CRITICISM OF THE ASTRONOMICAL 

 THEORY OF THE ICE AGE. 



T N a communication to the British Association at 

 ••■ Oxford, 1 gave an outline of a method of obtaining 

 a limit to the direct effect on terrestrial temperature of 

 the diminished winter sun-heat during epochs of great 

 eccentricity, the conclusion being that that effect had 

 been enormously exaggerated, and that the astronomical 

 theory of the Ice Age was really but a vague hypothesis, 

 having no sound physical foundation. 



It will be remembered that Dr. Croll's theory is shortly 

 this : In the long northern winters in the time of great 

 eccentricity, far less sun heat is received than at present ; 

 the direct effect of this decrease in sun-heat is a pro- 

 portionate decrease in terrestrial temperatures, or, more 

 properly, a proportionate decrease in the excess of 

 terrestrial temperature over the temperature to which 

 the earth would fall in the absence ol all sun-heat. So 

 far Croll and Sir Robert Hall, the later expounder of the 

 theory, agree. Hut now they part company. Croll 

 affirms that the lowering of temperature thus calculated 

 would be quite insufficient, and that it is the indirect 

 effect of this fall of temperature (chiefly the effect in 

 disturbing oceanic circulation) which gives rise to the 

 additional lowering of temperature necessary for the 

 production of an Ice Age. liall, on the other hand, 

 affirms that the direct lowering of temperature due to 



NO. 1306, VOL. 51] 



diminished sun-heat is ainply sufficient to cause an Ice 

 Age. I use the word affirms advisedly, because neither 

 writer assigns any reason. Apparently Croll's reason 

 was that he thought he could see additional causes, which 

 if they existed must have contributed to the effect, and 

 also that previous writers had said that the direct ett'ect 

 of the change in sun-heat would not be sufficient ; while 

 Ball seems to have considered that he had strengthened 

 Croll's argument so much that the new form of the 

 theory was as strong without the ocean currents, as 

 Croll's was with ocean currents. It does not seem to 

 have occurred to either writer to ask what change in 

 temperature would be necessary in order to produce an 

 Ice Age, so that they might see if the cause they assigned 

 would be sufficient ; yet one would have thought this was 

 the first step towards formulating a theory. 



The point in reference to which the two authors 

 employ numerical calculation is in obtaining the fall of 

 terrestrial temperature due to a reduction of sun-heat. 

 The problem is, of course, very complicated, and one 

 would expect that the most approved principles of physics 

 would be employed. Not at all. The physics is founded 

 on an incidental remark of the astronomer Herschel in 

 his " Outlines of .Astronomy" (edition of 1869), where he 

 assumes that the radiation of a body in space is propor- 

 tional to its absolute temperature. Yet it has for many 

 years been known to physicists that the radiation 

 increases faster than the temperature, and in iSSo or 

 iSSi what is now known as Stefan's law was published, 

 namely, that the radiation increases as the fourth power 

 of the absolute temperature. This would make an 

 enormous reduction in the calculated fall of tempera- 

 ture due to a diminished supply of heat — it 'uiouid reduce 

 it to o)ie-fotirtIi of the amount obtained on the erroneous 

 assumption employed by Croll and Ball alike. For if 

 temperature be solely due to sun-heat, the heat radiated, 

 say Afi\ where 6 is the absolute temperature, must be 

 equal to that received, say S, or 



Ae* = S, 

 hence 



d&_^ I e 



rfS 4 s' 



whereas the law of direct proportionality assumed by 

 Herschel, and adopted by Croll and Ball, gives 



,/» _ 9 

 .I'S S' 



a result four times as great as that obtained above — 



Turning now to Croll's form of the argument, we find 

 one very remarkable inconsistency, which I think is no 

 bad illustration of the special pleading which character- 

 ises that ingenious writer. When, in the first place, he 

 desires to show how great may be the midwinter fall in 

 temperature due to diminished sun-heat, he thus employs 

 the argument I have criticised above : — 



Let T,, be the present excess of midwinter temperature 

 at the latitude of the British Isles above the temperature 

 of space, i.e. above the temperature to which the earth 

 would fall if all sun-heat were to cease, and S,. the 

 quantity of sun-heat at present received on that 

 latitude on Midwinter Day, and let T^, and .S_,, be the 

 corresponding quantities for the supposed glacial winter. 

 Then, on Herschel's hypothesis, T,. is to T^, as S,. is 



to S». Having 



that way got an enormous fall of 



temperature, Dr. Croll goes on to say that a vast pro- 

 portion of our midwinter temperature in these isles is 

 due, not to sun-heat received by us, but to heat carried 

 to us by ocean currents. These ocean currents, he 

 argues, will be diverted in the supposed glacial period, 

 and thus there will be a/«r//;crgreat fall in temperature. 

 The argument for this double diminution of temperature 

 is, of course, utterly invalid. If a great proportion of our 

 winter-heat be not due to sun-heat, then a considerable 



