3i6 



NATURE 



\yuly II, 1879 



I 



a sulphur- crested cockatoo {Cacalua galerita) from Australia. 

 Soon after my arrival iu England I had occasion to cut his wing, 

 and this destroyed al! his former friendly feelings towards me. 

 On my removal from Blacldieath to Dublin, I placed him for a 

 few weeks in the Zoological Society's Gardens, Regent's Park. 

 Being in London in May I brought him with me on my return to 

 this "city. I went to the Gardens for him myself, and was 

 interested and somewhat pleased to find, on speaking to him, 

 that he had apparently forgotten me. On my way, in a cab, to 

 the hotel where I was staying, he was very friendly ; but on my 

 arrival there, as soon as I took off my hat, it was evident that he 

 recognised me, for his old manner at once returned. On arrival 

 here he appeared to remember my children, and resumed his former 

 friendliness towards them, but he still regards me as his enemy. 

 17, Leinster Square, Ratlunines, Dublin S. J. Whitmee 



Proceedings of the Aberdeenshire Agricultural 

 Association, 1878 



Your article (vol. xx. p. 288) touches a subject of vital 

 importance to the farmer in these bad times, and I hope 

 yon will follow it up by an appeal to Mr. Lawes and to the 

 Royal Agricultural Society of England to institute a parallel and 

 independent course of experiments, in order to test the conclu- 

 sion in regard to phosphalic manures announced by the Scotch 

 Association. 



The possible presence of humic, carbonic, and other acids in 

 more than usual proportion in Aberdeenshire soil and water has 

 often occurred to me as accounting for the similarity of results 

 behveen those local experiments of soluble and insoluble phos- 

 phates, but it can hardly be denied that a case has been esta- 

 blished for further experiments. The fact that acid and manure 

 makers have a direct pecuniary interest in the existence of a 

 prejudice in favour of soluble phosphate might alone have led 

 farmers to require some evidence before spending their money 

 under such scientific dictation, -but now that they have three 

 years' experiments testifying distinctly against the doctrine, they 

 may surely look to their advisers for something more than bare 

 assertions on authority that a "mineral phosphate is of little or 

 no value as a manre until it has been rendered soluble by acid 

 in course of manufacture." Alfred S. Jones 



Wrexham, July 27 



Spicula in Helix 



Whilst dissecting, a few days ago, a common garden snail 

 {Helix aspersa), I came across two calcareous spicula, lying 

 immediately under the " albuminiparous gland," which 1 cannot 

 find mentioned in any of the text-books. I at once dissected three 

 other snails of the same species, and in two I found no spicula, 

 while in the third I found one lying in the same place as the two 

 before-mentioned. Can any reader of Nature tell me whether 

 these are of the same nature as the diffused spicula of Doris, or 

 if not, of what nature they are ? They could not have been 

 "spicula amoris" (Huxley) of the .dart-sac, as they were not 

 contained in any sac at all, but were lying free in the above 

 position. Edwd. B. Tari-itt 



3, Waterfield Terrace, Blackheath, S.E. 



GENERAL RESULTS OF EXPERIMENTS ON 

 FRICTION AT HIGH VELOCITIES MADE IN 

 ORDER TO ASCERTAIN THE EFFECT OF 

 BRAKES ON RAILWA V TRAINS^ 

 II. 



SOME special experiments were made with blocks of 

 small area. The brake-blocks generally.used in these 

 experiments w^ere 12 inches long, by 3 inches wide, giving 

 a surface of 36 square inches; the small brake-blocks 

 were made so as to afford a surface of pressure against 

 the wheel of only one-third of this amount, or 12 square 

 inches, thus making the pressure per square inch three 

 times as great as before. The diminution of surface was 

 obtained by casting projections upon the face of the 

 block. The author is not prepared to say that any greater 

 coefficient of friction was obtained by the extra pressure 

 per square inch, although in one of the experiments, at a 

 velocity of sixty miles an hour, the rotation of the wheels 

 was arrested by these blocks, whilst this effect had not 

 been produced at that speed in other experiments. The 

 * Continued frjm p. 295. 



experiments on this form of block were stopped because 

 the blocks were entirely worn down in the course of about 

 twelve experiments. 



Mr. Rennie showed ' that high pressures per square 

 inch produced a greater coefficient of friction between 

 surfaces either moving very slowly or nearly at rest ; but 

 it must be borne in mind that the author's experiments 

 were made with high velocities, whereby a serious ele- 

 ment of disturbance is introduced, viz., the grinding away 

 of the surface ; and it is therefore probable that the in- 

 crease in the coefficient of friction due to increased pres- 

 sure, may have been neutralised by the lubricating effect 

 of the fine particles ground off the surfaces. 



While no certain opinion can be expressed as to the 

 relation which the coefficient of friction bears to pressure, 

 so far as these experiments are concerned, it is quite clear 

 that in proportion as the pressure is increased or dimi- 

 nished so will the actual friction obtained be increased or 

 diminished. When the friction which exists between the 

 brake-blocks and the wheel reaches a certain point, the 

 wheel ceases to rotate, and becomes fixed. This point is 

 reached when the frictional resistance of the blocks exceeds 

 the adhesion between the wheel and the rail if the speed is 

 kept up ; or, if the speed is slackening, when it exceeds the 

 adhesion between the wheel and the rail, plus the effort 

 required to retard the rotation of the wheel equally with 

 the retardation of the train ; and the excess of resistance 

 then acts as an unbalanced force, tending to destroy the 

 momentum of the wheel. 



Usually there are in a train a certain number of vehicles 

 braked and a certain number unbraked. If the brakes 

 acted on all the wheels, then the rotating momentum of 

 the wheels does not add to the distance in stopping a 

 train, because that momentum can be acted upon by the 

 brakes directly, without in any way affecting the adhesion 

 of the wheels to the rails. It simply requires an addi- 

 tional amount of brake-block pressure. 



With the unbraked portion of a train the rotating mo- 

 mentum of the wheels is an addition to the momentum 

 due to the weight of the train (including therein the actual 

 weight of the wheels), which cannot be utilised for 

 retardation ; and it therefore seems important that there 

 should be brakes on every wlieel of a train. 



As it thus appears that it is the adhesion which governs 

 the retardation which the brake-blocks can exert u^ on 

 wheels, it is manifest that the pressure brought to act on 

 the brake-blocks should never give an amount of friction 

 which exceeds the adhesion. At- a high speed, however, 

 the pressure required to produce a degree of friction equal 

 to the adhesion is much greater than what is required at 

 a low speed. 



The following table gives approximately the proportion 

 which the pressure to be applied to the brake-blocks 

 should bear to the weight upon the braked wheels, with 

 coefficients of adhesion between wheel and rail varying 

 from "30 to "IS of the weight on the wheels :— 



Jiatio of Brake-Block Press lire to Weight oh Wheels 



P/iil. Trans, for 1819, p. 159- 



