3i8 



NATURE 



{August 2, 1888 



method is described in a text-book, its advantages will 

 become more generally known. 



Mr. Brough deserves much praise for the care with 

 which he has searched European and American publica- 

 tions so as to bring his work up to date, and there is 

 little call for censure save upon minor points which do not 

 affect the general value of the text-book. 



It is time that some one should enter a protest against 

 two of the technical terms defined by the author, and 

 frequently met with in the reports of mining experts, viz. 

 " country rock " and " gangue." To say " country rock ' 

 is tautology. The word " country " alone, as used in 

 Cornwall, means "surrounding rock" or "enclosing 

 rock," and, if the provincialism is to be adopted, there is 

 no necessity to add the word " rock." The word " gangue " 

 is objectionable, because it has come to us through 

 Frenchmen, who apparently did not thoroughly understand 

 the meaning of the German word " Gang." " Matrix," 

 " lodestuff," and " veinstuff " are better words than 

 "gangue," which might well be allowed to drop out of 

 mining books, especially as it is rarely heard at mines. 



To cite the china clay deposits of Cornwall as examples 

 of stockworks is unfortunate, because the occurrence in 

 them of veins bearing workable quantities of tin ore is the 

 exception, not the rule. 



In Chapter VIII. Mr. Brough says: "In 1798 

 Breithaupt, of Cassel, invented a mine-surveying instru- 

 ment, which he called an astrolabium." This remark is 

 not correct, for, as the author well knows, the astrolabe 

 was invented by the ancients. The statement should 

 have been that H. C. W. Breithaupt was one of the first 

 to put an astrolabe upon a stand and use it for surveying 

 underground. According to Mr. Brough the theodolite 

 has been employed more or less for mine surveys since 

 1836. This date is probably correct as far as Germany 

 is concerned ; but as a matter of fact a mining theodolite 

 was supplied to the Imperial Brazilian Mining Association 

 four years earlier. 



The description of Prof. Borcher's method of using 

 magnets for ascertaining the precise line in which one 

 should continue to work in order to connect two drivages 

 in opposite directions which are approaching each other, 

 is not so clear as it ought to be. Mr. Brough omits to 

 explain, in reference to Fig. 101, that by construction the 

 points A, B, and C are situated upon the circumference 

 of a circle, the centre of which is E ; and the confusion 

 is increased by the statement that the triangle A E C is 

 " equilateral," whereas it is really only isosceles. The 

 consequence is that the reader is very much puzzled. 



However, these and a few other errors can easily be 

 corrected in a second edition, which is likely to be 

 required before many years are past ; because, as soon 

 as the book becomes known, no English-speaking mine- 

 agent or mining student will consider his technical library 

 complete without it. C. Le Neve Foster. 



OUR BOOK SHELF. 



Charles A. ding's Tours and Excursions in Great 

 .Britain. By Stephen F. Smart. (London : United 

 States Exchange, 1888.) 



This book is intended in the first instance for Americans, 



but it may also be of some service to English tourists. 



Taking London as a central point — " not only because it is 



the most notable city of the world, but because it is the 

 Mecca, if not the El Medina, of trans-Atlantic tourists, 

 at least " — the author describes a series of excursions, 

 any one of which will well repay the trouble of those who 

 may elect to follow his guidance. He also describes 

 various tours in Wales and Scotland. Mr. Smart has 

 been at pains to make himself familiar with the ground 

 over which he undertakes to lead others, and the infor- 

 mation he presents, so far as we have been able to test 

 it, is thoroughly trustworthy. Of course, no one who- 

 wishes to obtain a full account of any particular town or 

 district will think of consulting this little book. But 

 as a general sketch, it has considerable merits ; and it 

 will doubtless help many American visitors to make the 

 most of a brief visit to Great Britain. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take to return, or to correspond with the writers of, 

 rejected manuscripts intended for this or any other part 

 of Nature. No notice is taken of anonymous communi- 

 cations, .] 



The Supply of Bait for Sea-Fishermen. 



One of the first questions of practical importance with which 

 the Marine Biological Association has to deal is that of supply- 

 ing the long-line fishermen with a continuous supply of bait at 

 a cheap rate. Great distress is often occasioned through fisher- 

 men being unable to get the necessary bait for their long lines. 

 Mr. Kobert Bayly, of Plymouth, a governor of the Marine 

 Biological Association, has generously given a sum of .£500 to 

 be spent on investigations on the bait question, and the Council 

 have instructed me, as Director of the Association, to consider 

 the best means of spending this sum. I shall therefore bs glad 

 to receive any suggestions from gentlemen who may interest 

 themselves in this question, or to consider the work of any 

 investigator already in the field, with the view of employing 

 a suitable person to carry out a series of observations and 

 experiments. 



Two methods appear to offer a solution to the question. Either 

 the animals used commonly as bait, such as whelks, mussels, and 

 squid, may be reared artificially and kept in confinement till re- 

 quired, or some artificial bait may be invented which will lure 

 the more valuable kinds of fish to the hook. 



The former of these methods has been successfully practised in 

 France, but such is the operation of the English laws on shore 

 fisheries that there is very little prospect of its being possible in 

 England, unless those laws are altered. 



The second method, though more apparently difficult, is the 

 more likely to attain success. Fish are undoubtedly guided by 

 smell and taste in the selection of their food. Some are known 

 to be very nice about the kind of food offered to them, and will 

 only take certain kinds of bait. The whelk is a very favourite 

 morsel, and has a distinct smell and taste : it may be possible to 

 determine by analysis the essential oil or whatever it may be 

 that gives this odour, and to imitate it sufficiently well to deceive 

 the fish. The trade is able to imitate successfully the bouquet 

 of wines : cannot chemistry produce an imitation of the bouquet 

 of the whelk ? G. C. Bourne. 



The Laboratory, Citadel Hill, Plymouth, July 31. 



Geometric Meaning of Differential Equations. 



In the Proceedings of the Royal Asiatic Society of Bengal, 

 1888, p. 76, Prof. Asutosh Mukhopadhyay has proposed a really 

 excellent mode of geometric interpretation of differential equa- 

 tions in general : viz. writing the equation in form F = 0, the 

 geometric meaning of the symbol F considered as a magnitude 

 (angle, line, area, &c), in any cui-ve whatever (wherein F is of 

 course not zero), is, if possible, to be formed ; then the geo- 

 metric meaning of that equation obviously is that the quantity F 

 vanishes right round every curve of the family represented. 

 This is the most direct geometrical interpretation yet proposed. 



