April lo, 1884] 



NATURE 



547 



tials which appeared in these columns a short time ago, 

 but with some additional matter, must, from the clear 

 explanation of the principles involved in the different 

 methods of measurement, take a high position as an 

 educational uork, and, from the care with which details of 

 manipulation are in many parts described, form a valuable 

 laboratory guide. 



The author begins by explaining Gauss's method of 

 finding the horizontal iniensity of the earth's magnetism. 

 Instead of describing an "instrument-maker's" magneto- 

 meter, and showing how with this expensive luxury H 

 may be determined, he gives simple, clear, and full direc- 

 tions for constructing, with such common materials as 

 are to be found in any laboratory, all that ii necessary for 

 making this determination with great accuracy. 



A description of the tangent galvanometer in some of 

 its forms and an explanation of some of the units natur- 

 ally follow. Here, by treating each unit separately with 

 many illustrations depending on the aspect from which 

 they are viewed, the author has succeeded in giving them 

 a reality which students often find it difficult to believe 

 they possess. 



The next two chapters are devoted to a description of 

 the construction and graduation of Sir W. Thomson's 

 " Graded Galvanometers." These instruments possess 

 so great a range, and are, when used carefully in the labo- 

 ratory, so accurate and convenient, though rather delicate 

 for an engine-room, that an exact description from head- 

 quarters of their construction, of the precautions which 

 must be observed in their use, and of the means of gra- 

 duating them is especially valuable. 



The various methods employed in measuring any 

 resistance from that of a thick copper rod to that of a 

 piece of gutta-percha are given, and in many cases 

 explained by nimierical examples. 



The methods by which the energy due to direct or to 

 alternating currents may be measured is explained — in the 

 latter case on the assumption that the current strength 

 varies harmonically with the time. 



The chapter on the measurement of intense magnetic 

 fields is especially interesting, for the methods given, 

 depending on the use of suspended bits of wire attached 

 by threads to pendulum weights, or equally simple and 

 easily contrived devices, show how the experimenter may 

 in many cases be independent of the elaborate work of 

 the instrument-maker. C. V. B. 



Field and Garden Crops of tlic North- Western Provinces 

 and Oiidii. By J. F. Duthie, B.A. F.L.S., Superin- 

 tendent of the Saharanpur Botanical Gardens, and J. 

 B. Fuller, Director of Agriculture, Central Provinces. 

 Part 2. With Illustrations. 



As a work of reference it will be very valuable, for it 

 contains well-arranged details of some of the more im- 

 portant crops under cultivation, and the information is 

 well and systematically arranged. Care has been taken 

 in each case to secure a complete but still a concise 

 statement, which is sufficient to guide the cultivator in all 

 the specialities of management necessary to secure suc- 

 cessful results. A good drawing illustrates each crop 

 treated of, and its several cultivated varieties, and with 

 these we have carefully-prepared descriptions of each 

 plant in succession, and its general history. The districts 

 within w'hich the cultivation can be successfully extended 

 are also set forth with great clearness and precision. For 

 accuracy of details, in a very accessible form, this work 

 leaves little to be desired. 



A Treatise on Higher Trigonometry. By the Rev. J. B. 



Lock. (Macmillan, 1884.) 

 This is the promised complement to the same writer's 

 " Treatise on Elementary Trigonometry," which we 

 noticed very favourably in these pages at the time of its 

 appearance (vol. xxvi. p. 124}. It is concerned principally 



with series, the errors which arise in practical work, and 

 the use of subsidiary angles in numerical calculations. 



A short chapter on the use of imaginaries is justified 

 by the position this subject holds in the London Uni- 

 versity Examinations, and no apology is needed for the 

 space assigned to an account of, and a collection of exer- 

 cises upon, the hyperbolic sine and cosine. We have read 

 the text carefully, and though almost of necessity there 

 are numerous typographical mistakes, only one or two 

 (for 2 <7 cos 2 6, p. 127, line 3, read a cos 2 6) will incon- 

 venience a student. In addition to the numerous 

 examples in the text, there are fourteen specimen papers 

 from Cambridge and other examinations. 



The only article to which «e take exception is § g, the 

 proof of which may be, if we mistake not, considerably 

 simplified. The book can be confidently recommended 

 to the use of advanced pupils in our schools, and will 

 meet the wants of most students in our Universities. 



LETTERS TO THE EDITOR 



[ The Editor does not hold liimselj responsible for opinions expressed 

 by his correspondents. Neither can he undertate to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonytnons communications . 



[The Editor urgently requests con espondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of comii.unications containing interesting and novel facts. '\ 



Teaching Animals to Converse 



You did me the honour some weeks ago (January 3, p. 216) 

 to insert a letter of mine, containing suggestions as to a method 

 of studying the psychology of animals, and a short account of a 

 beginning I had myself made in that direction. 



This letter has elicited various replies and suggestions which 

 you will perhaps allow me to answer, and I may also take the 

 opportunity of stating the progress which my dog "Van" has 

 made, although, owing greatly no doubt to my frequent absences 

 from home, and the little time I can devote to him, this has not 

 been so rapid as I doubt not would otherwise have been the case. 

 Perhaps I may just repeat that the essence of my idea was to 

 have various words, such as "food," "bone," "water," "out," 

 &c. , printed on pieces of cardboard, and after some preliminary 

 training, to give the dog anything for which he asked by bringing 

 a card. 



I use pieces of cardboard about 10 inches long and 3 inches 

 high, placing a number of them on the floor side by side, so that 

 the dog has several cards to select from, each bearing a different 

 word. 



One correspondent has suggested that it would be better to 

 use variously coloured cards. This might no doubt render the 

 first steps rather more easy, but, on the other hand, any temporary 

 advantage gained would be at the expense of subsequent diffi- 

 culty, since the pupil would very likely begin by associating the 

 object with the colour rather than with the letters ; he would, 

 therefore, as is too often the case with our own children, have 

 the unnecessary labour of unlearning some of his first lessons. At the 

 same time the experiment would have an interest as a test of the 

 condition of the colour-sense in dogs. Another suggestion has been 

 that, instead of words, pictorial representations should be placed on 

 the cards. This, however, could only be done with material 

 objects, such as "food," "bone," "water," &c., and would not 

 be applicable to such words as "out," "pet me," &c. ; nor even 

 as regards the former class do I see that it would present ar*y 

 substantial advantage. 



Again, it has been suggested that " Van " is led by scent 

 rather than by sight. He has no doubt an excellent nose, but in 

 this case lie is certainly guided by the eye. The cards are all 

 handled by us, and must emit very nearly the same odour. I 

 do not, however, rely on this, but have in use a number of cards 

 bearing the same word. When, for instance, he has brought a 

 card with " food " on it, we do not put down the same identical 

 card, but another with the same word ; when he has brought 

 that, a third is put down, and so on. For a single meal, there- 

 fore, eight or ten cards will have been used, and it seems clear, 

 therefore, that in selecting them "Van" must be guided by the 

 letters. 



