June 4, 1874] 



NA TURE 



85 



has not clone or thought of doing — f>rci'i: the proportionality of 

 cause and effect. Neither he, nor any other of Mr. Spencer's 

 opponents, has made the smallest attempt to deal with this main 

 issue. Mr. ."Spencer alleges that this co;;nition of proportionality 

 is A priori : not in the old sense, but in the sense that it grows 

 out of experiences that precede reasoning. His opponents, 

 following Prof Tait in the assertion that Physics is a purely 

 experimental science, containing, therefore, no d priori trntlis, 

 affirm that this cognition is h posteriori — a prodnct of conscious 

 induction. Let us hear what are the experiments. It is required 

 to establish thetrnth that there is proportionality between causes 

 and eflects, liy a prooas which !io:ohorc' nssiuncs that if one unit of 

 force prodnces a certain unit of effect, two units of such force 

 will produce two units of such effect. Until the "Senior 

 Wrangler " has done this he has left Mr. Spencer's position 

 untouched. 



Bayswater, May 20 James Collier 



The Great Ice-Age 



In reply to Mr. Belt's letter (p. 62), I did little more than 

 express an adverse opinion to his theory, because to discuss it 

 would have required an es=ay. I expressed this because I 

 notice that unless something like a demurrer is entered against a 

 new theory it is apt to be taken for granted in subsequent text- 

 books and papers written by those who have had no oppor- 

 tunities of obtaining a practical knowledge of the subject. For 

 the above reason I must answer his strictures very briefly. 



(i) I fail to see w'ny the Scandinavian sea-beaches are irre- 

 levant. (2) I have more than once read Mr. Tiddeman's paper, 

 and without committing myself to all its conclusions, think I 

 may quite it as assuming that the Lake district (as distinguished 

 from North Lancashire) was the centre of a great ice sheet ; not 

 that it was over-ridden by ice coming from somewhere further 

 north. The same might be expected to be the case with the 

 Welsh mountains ; and Mr. W. Kingsley has brought forward 

 good evidence of the existence of an ice-sheet there also. (3) 

 Mr. Belt appears to forget that shells have been found not only 

 at Moel Tryfaen, but also near Llyn Ffynnon-y-gwas, about two 

 miles west of the peaks of Snowdon. Does Mr. Belt mean to 

 say that Snowdon could not protect itself in the heart of its own 

 domain better than this ? If the Lake mountains had au ice- 

 sheet, surely .Snowdonia ? Mr. Belt asks for evidence of the 

 shore of the glacial sea. I reply that to me these and the Moel 

 Tryfaen beds, not to mention others, appear to be far more prob- 

 ably littoral deposits than transported. For example, I think 

 it in the highest degree improbable that the Vale Royal shells 

 (Lyell, "Antiquity of Man," p. 317) could be brought to their 

 present position (more than 1,100 feet above the sea) by any ice- 

 sheet without the cjld being enough to cover nil the higher 

 ground in Britain with ice, and so protect it. I did not deny a 

 glacier might push a stone before it up-hill ; ray contention w.as 

 that the enoimous force which would be exerted on beds scooped 

 out as described, and shoved some 1,500 feet up-hill for miles 

 over broken ground, would crush the shells to a far more com- 

 minuted state than they are now in. With regard to Holderness, 

 Mr. CroU's view of the shells there appears to me to be at present 

 only a theory of which Mr. Searles V. Wood, jun. , has effectually 

 disposed (Geol. Mag. 1872). I grant there are some difficulties in 

 the submergence theory ; my position is that those in Mr. Belt's 

 are very much greater. 



A recent perusal of Mr. J. Geikie's suggestive book, the 

 "Great Ice Age," has brought before my mind more strongly 

 than ever a dilemma, which, as it appears to me, the modern 

 school of Glacialists cannot escape. 



He speaks of the till as a gruiid moriine or moraine frofonde 

 formed between the glacier and the rock, while he attributes the 

 majority of rock-basins to the action of the glaciers. Now it 

 appears to me that if the glaciers could pass over considerable 

 deposits of \.\\\% moraine p) olo}ide vi'AVavA sweeping it clean away, 

 then their action as erosive agents must have been comparatively 

 feeble ; or, if they could scoop out great rock basins like the 

 Alpine and (buried) Highland lakes, then they would have peeled 

 off almost all the till from the land. As it appears to me, the 

 analogy with a river, by means of vi-hich Mr. J. Geikie (p. SS) 

 seeks to escape from a portion of this difficulty, does not hold. 

 When a river begins to deposit sand and gravel largely, its work 

 as an erosive agent at that place is almost over. Besides we 

 cannot conceive a nearly solid mass, like a huge glacier, changing 

 its motion so rapidly as a stream of water. Difficult as it un- 

 doubtedly is to explain some of the lake-basins, it appears to 

 me that the great bulk of his eviden;e, with regard to till and 



other deposits over which ice-streams have passed, shows how 

 slightuiider ordinary circuiustanc;s is their erosive power i and 

 this has been confirmed by every journey that I have made among 

 the Alpj. I may add also that from study of the same regions 

 my faith in a moraine pro/onds is much shaken. I believe that, 

 except possibly as a very local and exceptional phenomsnon, it 

 exists solely in the imaginationof the eminent geologists of whom 

 Mr. Geikie is a disciple. T. G. Bo.'^NEY 



St. John's College, Cambridge, May 26 



Photographic Irradiation 



In the paper referred to by Prof. Forbes (N'.VTURU, vol. x. 

 p. 29) what is ordinarily called Photographic Irradiation was 

 attempted to be explained by us, not as being caused by re- 

 flections from the back of the plate, but as biing due to th; su n 

 of all the optical imperfections of the instrument with w'.itcli 

 the photograph is taken. 



If Mr. Stillman(p.63) will refer to our original paper, published 

 in the Monthly Avtiees for June 1S72, he will find tliat only the 

 cloudy indefinite haze which surrounds the image of a luminous 

 object, and which has frequently been called halation, was re- 

 ferred by us to reflection from the back of the plate. 



When an over-exposed photograph is taken upon an opaque 

 plate a marked fringe of irradiation still remains, and expsri- 

 ments were instituted by us which appeared to show that this is 

 not to be accounted for by any circulation taking place within the 

 thickness of the collodion or by the chromatic dispersion of the 

 lenses ; but when the oblique pencils from the edges of the 

 lenses were stopped out the irradiation fringe was found to be 

 greatly decreased. We were led to conclude that irradiation 

 is to be accounted for by the {.\ct that each luminous point in the 

 object is not accurately represented by a hininous point in the 

 image, but rather by a luminous patch of sensible area, the 

 central and more intense portion of which prints itself first in the 

 photograph, giving coaiparatively sharp picture prints when the 

 exposure is short ; but as the picture is still further exposed, the 

 outer portions of the luminous patches imprint themselves, and 

 by their overlapping cause the blurred appearance to which has 

 been given the name of irradiation. LiNDS.w 



A. COWPER R.VNVARD 



Uncompensated Chronometers and Photographic 

 Irradiation 



With regard to the employment of uncompensated chro- 

 nometers (N.\TURE, vol. X. p. 63), I have every reason to believe 

 that the Russians alone have tested them. For some reason 

 which is not easily discovered, the employment of a negatively 

 compensated chronometer has not given any very remarkable 

 results. The Russians have employed simply an uncompensated 

 chronometer ; and have obtained very remarkable results as men- 

 tioned in my article on the Transit of Venus to which Prof. 

 Everett has alluded. 



With regard to the prevention of photographic irradiation, of 

 course various means have been employed for dry plates ; but I 

 believe that Lord Lindsay and JMr. Ranyard were the first to 

 experiment on the matter exhaustively. I believe Mr. Stillman 

 would be interested in reading their paper in the Monthly 

 Notices. At the same time all honour is due to the photo- 

 graphers named by him for their experiments. 



Birkenhead, June i George Forbes 



The Seal Fishery 

 CaI'T. David Gray, of the steamship Eclipse, has done good 

 service to the cause of humanity in writing, and Mr. Buckland 

 in publishing, the letter on the seal fishery which appears in 

 Land and IVater for May g. The fearful cruelties perpe- 

 trated year after year, and the enormous waste of life entailed by 

 the reckless manner in which the seal fishery is prosecuted, are 

 well known, but no steps have hitherto been taken to regulate a 

 trade which, if carried on within proper bounds, would continue to 

 yield great profits, but if still pursued with such utter disregard 

 to consequences must soon end in the extermination of ihe whole 

 race. As an instance of the wastefnliiess of the mode of proceed- 

 iig, Capt. Gray says that five .-hips attacking a pack of sea's, 

 in four d.ays killed about 10,000 oia seals ; " add 20 percenr. for 

 seals mortally wounded and lost, gives .an aggregate of 12,000 

 old ones ; add 12,000 young which died of starvation, gives 

 24,000 ; but this is not all. The men spread on the ice, so that 

 the old ones that were left alive could not get on to suckle their 

 young. The consequence was that the whole of the young 



