366 



NATURE 



[Feb. 1 6, I 



wishes to substitute for Latin one of the sciences enumerated in 

 Group II., it should be allowed a maximum of 3000 marks. 

 2 Powis Square, W. Henry Palin Gurney. 



•' British and Irish Salmonidse." 



As your reviewer allows that he " intentionally omitted " five 

 words from a sentence of mine which he quoted in order to 

 criticise, I may well leave comments on such a proceeding to 

 your readers. I willingly acquit him of having purposely made 

 me to suggest utter nonsense, as I cannot help thinking that his 

 knowledge of fish-culture was such that he was unaware he was 

 doing so. 



As to the second point he says, " I doubted and still doubt 

 if there is any method practised in which layers of moss are 

 used and are separated from the eggs by muslin and similar 

 material." As he rejects the Howietoun account which I gave, 

 I now submit extracts from two standard works, one American, 

 the other English, which will, I believe, be conclusive to those 

 who are ignorant of fish-culture, for every fish-culturist is 

 aware that this plan is commonly adopted. Livingstone-Stone 

 (" Domesticated Trout," ed. 3, 1877) remarked : — " Theodore 

 Lyman recommends placing each layer of eggs in a fold of 

 mosquito netting to keep them from mixing with the moss and 

 so facilitate the unpacking of them. This is a great improvement. 

 By all means use mosquito netting" (p. 149). Mr. Andrews, 

 of Guildford, wrote thus in the Badminton Series (" Salmon 

 and Trout," 1885) : — "The plan of packing does not vary much 

 with trout breeders. The eggs are placed in alternate layers 

 between moss, and protected by a covering of mosquito netting, 

 muslin, swans' down, calico, or butter cloth, so arranged that the 

 eggs shall not be crushed or escape " (p. 447). 



As regards the third point, your reviewer now appears to be 

 convinced that Saltno namaycush is a char, as I stated it to be. 

 It must be a matter of regret that he omitted to investigate the 

 foregoing questions prior to authoritatively writing upon them in 

 such a well-known publication as Nature. 



Cheltenham, February 4. Francis Day. 



In his last letter Mr. Day has certainly proved the correctness 

 of the statement in his book that salmonoid eggs are packed 

 with layers of moss from which they are separated by muslin or 

 other textile fabric. If I had known as much about salmon- 

 culture as he, I certainly should not have questioned the state- 

 ment ; it is to be noted that I only questioned and did not deny. 

 If I had been as completely versed in the knowledge of Sal- 

 monidte as Mr. Day, I should have written a book on the subject 

 instead of reviewing his. But the essential point, which Mr. 

 Day seems incapable of appreciating, is this : that there was 

 nothing in the notes on the subject of packing in his book which 

 confirmed the statement in the text ; and although my doubts as 

 to the correctness of that statement are removed by his letter, 

 they were perfectly justifiable in a reader of his book. Mr. Day 

 does not apparently suspect that people interested in the subject, 

 including the reviewer, read his book for the sake of gaining 

 information, and not because they already know as much about 

 the subject as himself. All I had to do was to give my impres- 

 sions of the book as I found it : the fitness of my criticisms is 

 only the more established by the lengthening appendix to his 

 book which Mr. Day is now publishing in your correspondence 

 columns. Your Reviewer. 



MODERN VIEWS OF ELECTRICITY^ 

 Part III. Mk.Q,^-KX\^^i~{contimied.) 



VIII. 



TT will now be perceived that a fly-wheel in rotation is 

 •*• the mechanical analogue of magnetism, or more de- 

 finitely of a section of a line (or tube) of magnetic force ; 

 and that a brake applied to such a fly-wheel, with consequent 

 slip, dissipation of energy, and production of heat, is in 

 some sort a mechanical analogue of an electric current. 



The field is regarded as full of geared elastic vortices 

 or whirls, some of which are cogged together, so to speak, 

 while others are merely pressed together by smooth rims. 



' Continued from p. 348. 



It is among these latter that shp is possible, and in the 

 regions occupied by them that currents exist ; the energy 

 dissipated here being transmitted through tlie non-slippery 

 or dielectric regions from the source of power, just as 

 energy is transmitted from a steam-engine through mill- 

 work or shafting to the various places where it is dissipated 

 by friction. 



Mechanical Force acting Oft a Conductor conveying a 

 Current. 



In Fig. 41 the conducting portion is shown with opposite 

 rotations on either side of it. Now superpose a uniform 

 rotation all in one direction upon this, so as to increase the 

 spin on one side and diminish it on the other. Imme- 

 diately the extra centrifugal force on one side will urge any 

 movable part of the conductor from the stronger to the 

 weaker portion of the field. 



The field for a direct and return circuit may be similarly 

 drawn by superposition of their separate w'hirls (see Fig. 

 40) ; and so it becomes evident why a circuit tends to 

 expand so as to inclose the largest possible area, even if 

 no other magnetic field than its own be acting on it. 



Also if two circuits are arranged near each other in a 

 plane, with their currents in opposite directions, they will 

 more or less neutralize each other's effect on the space 

 between them, causing (if equal) a region of no spin there. 

 Their neighbouring portions will thus get urged together 

 by the unbalanced pressure on the other side : or, currents 

 in the same direction attract. 



Fig. 44. — Two parallel conductors conveying equal currents in one directioii 

 and getting pu'shed together by the centrifugal force of the outside 

 whirls, no whirl existing between them. The length of the arrows 

 again suggests the distribution of magnetism in the conductors. Fig. 4c 

 showed the correlative repulsion of opposite currents. 



As for the effect of iron introduced into a circuit, it 

 brings into the region of space it occupies some two or 

 three hundred times as many lines of whirl as were there 

 before, and these naturally contribute mightily to the 

 effects, both those exhibiting mechanical force and those 

 exhibiting inertia. 



When one says, as roughly one may do, that iron brings 

 300 fresh lines into the field, one means that for every 

 whirl otherwise excited, 300 more are faced round in 

 the iron. And this process goes on while the field is 

 increasing in strength until the total number of whirls in 

 the iron begins to be called upon ; when this point is 

 reached the rate of addition is not maintained, and the 

 iron is said to show signs of saturation. Ultimately, if 

 ever all its whirls were faced round, the iron would be 

 quite saturated ; but long before this point is reached 

 another cause is likely to make itself felt, viz. the falling 

 off in the strength of the whirls already faced round, by 

 the action of the strong magnetic induction, which is all 

 the time acting so as to weaken the iron currents so far 

 as it is able. And thus at a certain point hitherto un- 

 reached by experiment the iron may not only fail to- 

 increase the strength of the field any more, but may 

 actually begin to diminish it. 



