IS<!> 



NATURE 



[June i6, 1892 



pa^rent has indeed shown how a classification of minerals 

 according to their mode of occurrence may be employed 

 even in a systematic treatise ; but Dr. Hatch's more 

 humble attempt is not open to the criticism to which 

 an ambitious work on the same lines would obviously be 

 liable. It is clear that in a book of this kind there is not 

 much scope for originality of treatment, but Dr. Hatch 

 has admirably united brevity and clearness in his treat- 

 ment of the crystallographical and physical characters of 

 minerals. His method of giving the names and commonly 

 employed reference letters to the crystal-combinations 

 which he figures is well adapted to prepare the student 

 for consulting larger treatises on the subject. So, too, 

 the reference to the use of symbols, though it must evi- 

 dently be very slight in a work of the dimensions of that 

 before us, is eminently judicious. A short table of 

 symbols of the chief forms belonging to each system, 

 according to Miller and Naumann,will enable the beginner 

 to recognize the meaning of all the very commonly occur- 

 ring combinations ; and it is clearly inexpedient to attempt 

 more than this in such a very elementary work. We can 

 confidently recommend the book as an excellent sum- 

 mary of mineralogical science, adapted to the wants of 

 the geological student ; and we believe the perusal of this 

 small work may even be of advantage to those who 

 desire to enter upon the more systematic study of the 

 science of mineralogy. J. W. J, 



To the Snows of Tibet through China. By A. E. 



Pratt, F.R.G.S. (London : Longmans, Green, and 



Co., 1S92.) 

 The author of this book says in the preface that he 

 has done his best " to withstand the temptation to 

 generalize from limited experience, to which travellers in 

 China seem peculiarly liable." Yet in his last sentence 

 he expresses the opinion that several incidents he has 

 mentioned " will show what a credulous and cowardly 

 race the Chinese are." It ought surely to have occurred 

 to him, when he set down this harsh and rather foolish 

 judgment, that it was a striking example of the kind of 

 generalization which he had wished to avoid. Fortu- 

 nately the statement, although it seems to convey Mr. 

 Pratt's final impression of the Chinese people, does not 

 represent the general character of his work, in which 

 scientific readers will find a good deal to interest them. 

 He went to China in 1887 for the purpose of studying 

 the natural history of the country, and remained until 

 1890, fixing his head-quarters at Ichang, a town on the 

 left bank of the Yang-tze-Kiang, mo miles from its 

 mouth. He crossed the frontier of Tibet, and at Ta- 

 tsien-lu met Mr. Rockhill, whose excellent account of 

 travels in Tibet we lately reviewed. Mr. Pratt worked 

 hard in the various regions he visited, and collected 

 many valuable specimens in several departments of 

 natural history. He has not a very bright or attractive 

 style, but many of his facts are themselves so interesting, 

 and his enthusiasm as a collector is so keen and per- 

 sistent, that there are few passages which his readers 

 will desire to skip. In an appendix, Dr. Albert Giinther 

 gives a list of the species of reptiles and fishes brought 

 by Mr. Pratt from the Upper Yang-tze-Kiang and the 

 province Sze-chuen, with a description of the new species. 

 There are also lists of birds and of Lepidoptera. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of ^K'^V'&JS.. 

 No notice is taken of anonymous communications. \ 



Absolute Electrometer for Lecture Purposes. 

 I THOUGHT it might be welcome to some of your readers to be 

 made acquainted with the following simple and cheap instni- 



NO. I 1 8 1 , VOL. 46] 



ments which I have now used for some years with advantage in 

 lectures, and also for many scientific purposes. They are 

 electrometers, which are divided directly into volts. The 

 needle, which is made of aluminium, moves about a horizontal 

 axis of hard steel, and is repelled from the vertical brass piece 

 connected with the knob above. The instruments have the 

 advantage that they are much easier of manipulation than the 

 gold-leaf electroscope, while the sensibility is nearly the same, 

 and fully suffices for all lecture purposes. Potentials are read 

 off directly in volts, so that the measurements in the experi- 

 ments on electrostatics and electrodynamics can all be referred 

 to the same unit, whereby the conception of the student gains 

 1 in distinctness, and the lecture in simplicity. In consequence 

 of the specially careful workmanship, the needle adjusts itself 

 I quickly and with certainty, so that readings may be made to 

 about 10 volts. The back and front consist of glass disks 

 i I mm. thick, each of which covers a plate of zinc of the same 

 I size, out of which are cut two equal and opposite slits, through 

 ' which the position of the needle on the brass scale is read off. 

 The readings of the instrument are only correct when these 

 plates are in position. 



When the instrument is used in the lecture, the two plates are 

 taken away, and the back glass plate covered with tissue paper, 



the instrument being illuminated from behind. The deflections 

 are then easily visible in a room for more than a hundred 

 students. 



The method of graduation of these instruments I have 

 described in full in Wiedemann's Annalen, vol. xliv., 189 1, p. 

 771. They can be procured from the University mechanician 

 here, Herr Albrecht, in three different sizes, 0-1500, 0-4000, 

 and 0-10,000 volts. The first of these is the substitute for a 

 gold-leaf electroscope. Herr Albrecht also makes the instru- 

 ments for technical purposes. F. Braun. 



Physical Laboratory, Tubingen, May 28. 



Saturn's Rings. 



The writer of the " Astronomical Column," in your number 

 of June 2, directs attention to some observations of M. 

 Bigourdan on certain peculiarities in the appearance of the 

 following arm of Saturn's Rings observed by him on May 21. 

 He mentions in particular a protuberance situated near Cassini's 

 division. This, I think, is easily accounted for in a quite 

 different manner. At 9h. 6m. p.m., according to Marth's 

 ephemeris, two satellites, Enceladus and Tethys, were in con- 

 junction with the east end of the ring. They were going in 

 apparently opposite directions, Tethys away from Saturn. 

 Their conjunctions with the middle of the Cassini division 

 would, I find, take place at 8h, 36m. p.m. for Tethys, and at 

 9h. 36m. p.m. for Enceladus. Both satellites would be so 

 close to the ring as to appear inseparable from it. Tethys, 

 moving in an orbit inclined as much as 65' to the plane of the 

 rings, might easily be half superposed in appearance upon the 

 northern boundary of the rings. The following remarks are 

 from my observation-book of date May 21 :^ 



