244 



NATURE 



[January i6, 1896 



of the introduction of such a course of work as is herein 

 described. It encourages thought, creates interest in 

 chemistry, and furnishes the kind of knowledge most 

 Hkely to prove of advantage in after years. Not only in 

 organised science schools, but in every school where 

 chemistry is taught, the course described in this book 

 could be profitably introduced. 

 Observaciones de precisio7i con el Sextante. Por el Conde 



de Canete del Pinar, Cdpitan de Fragata Retirado. 



Pp. 180. (Madrid : Ricardo Alvarez, 1895.) 

 A DESCRIPTION of the sextant and the uses to which it 

 can be applied is here given in seven chapters, of which 

 the first describes the instrument, and shows how it 

 may be corrected. Following, we have four chapters on 

 different means of the determination of latitude by means 

 of stars, showing the methods trigonometrically, and also 

 giving examples. The accuracy of observations taken 

 by the sextant is graphically shown by two tables, giving 

 the latitude obtained on several successive days. Lastly, 

 we have a description of the means by which time is 

 determined, and also how the longitude is obtained by 

 means of the moon and stars. Throughout the book 

 there are numerous examples, and no pains have been 

 spared to make it useful. 

 First Stage Mechanics. By F. Rosenberg, M.A. Pp. 



296. (London : W. B. Clive, 1895.) 

 This book has been made to fit the requirements of the 

 elementary stage of theoretical mechanics of solids, as 

 laid down in the syllabus of the Department of Science 

 and Art. It is the first volume of a new series of Depart- 

 mental text-books, and it possesses all the characteristics 

 of the literature of the University Correspondence 

 College Press ; by which remark we mean that the text 

 is concise, the examples numerous, and the comparative 

 importance of the sections is indicated by the thickness 

 of the type in which they are printed. What more does 

 a student require, who is learning theoretical mechanics 

 for examinational purposes ? 

 The Story of the Solar System. By George F. Chambers, 



F.R.A.S. Pp. 202. (London : George Newnes, 



Limited, 1895.) 

 We are glad to be able to state that the twenty-eight 

 illustrations in this book are better than those in the 

 companion volume on the " Stars," by the same author. 

 Mr. Chambers has contrived to compress an immense 

 amount of information within a smai compass, and his 

 descriptions possess the double quality of simplicity and 

 attractiveness. We do not know of a book in which so 

 much is told about the solar system within such narrow 

 limits. 



British Gtiiana and its Resources. By the author of 

 " Sardinia and its Resources." Pp. 104. (London : 

 George Philip and Son, 1895.) 

 The question of frontier between British Guiana and 

 Venezuela is now so much to the front, that a large pubhc 

 will be interested in this description of the history, 

 features, and resources of the region in which the de- 

 batable land lies. The book will be found valuable not 

 only on this account, but because it is full of information 

 useful to visitors to British Guiana. Travellers of all 

 tastes and inclinations will find that the country offers 

 many attractions, and is as wide a field for observation 

 and collection as could be desired. 



Mammals of Land and Sea. By Mrs. Arthur Bell (N. 



D'Anvers). Pp. xii + 191. (London : George Philip 



and Son, 1896.) 

 Although this volume will assist its readers to know 

 the general characteristics of members of the mammalian 

 family, it possesses no novel features, and the illustrations 

 belong to a past age. Some readers may find the book 

 interesting, but few will pronounce it attractive. 



NO. 1368, VOL. 53] 



LETTERS TO THE EDITOR. 



\7'he 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 Nature. 

 No notice is taken of anonymous communications. ^ 



A New Method of Measuring Temperature. 



The recent publication of papers dealing principally with 

 thermometry, by Mr. Griffiths, and by the Kew Standardising 

 Bureau, has suggested to me that the publication of a new 

 thermometric method which I have used for some years may be 

 of use. 



Briefly, in this method two thermo-j unctions are used ; one is 

 placed, protected or not, as circumstances dictate, in the sul>stance 

 of which the temperature is to be measured, the other in the 

 bulb of an air or nitrogen thermometer. This junction is 

 blackened, and may or may not be protected, but should be in 

 the same state as the other. The bulb of the air thermometer 

 may or may not be silvered or platinised. Within the bulb ot 

 the air thermometer is placed a coil of platinum wire, in series 

 with this being a carbon resistance and a storage battery. The 

 bulb of the thermometer is protected by a layer of slag- wool, or, 

 if this cannot be obtained, of asbestos ; or a sheath of polished 

 metal may be used. In the thermo-electric circuit a low 

 resistance galvanometer is placed. I prefer to use a d'Arsonval. 



T'^t modus operandi If, as follows. The free thermo-j unction 

 is placed in the substance whose temperature is to be measured. 

 The galvanometer is immediately deflected. The circuit of the 

 platinum heating coil is then closed, and the carbon resistance 

 screwed down till the galvanometer needle comes back to zero, 

 or until making and breaking the thermo-electric circuit pro- 

 duced no movement of the needle. When this is the case the 

 temperature of the air or nitrogen in the bulb will evidently be 

 the same as that of the substance to Vje measured, and can be 

 directly read off" in any of the usual ways on the thermometer. I 

 prefer myself to use the constant volume method. 



It is necessary, of course, that the thermo-junctions be both in 

 the same physical state. This is generally secured with 

 sufficient accuracy by cutting the wire from the middle of a much 

 larger piece which has been well annealed. In connection with 

 other work I have found that two samples of metal, chemically 

 identical but having different rigidities and thermo-electric 

 powers, may always be brought to identical states by heating 

 for a time at white heat in vacuo, first introducing, if necessary, 

 oxygen or hydrogen to decompose any hydride or oxide com- 

 bined with the metal. I have never found it necessary to do 

 this in making thermo-junctions, but its use is recommended to 

 experimenters who are studying the physical properties ot 

 metals. 



The advantages of this method are as follows : 



( 1 ) No assumption is made in regard to any law of variation 

 of thermo-electric effect with temperature. 



(2) No assumption is made with respect to variation of voltage 

 of standard cell in relation to temperature. The error due to 

 the fact that the saturation of the sulphate solution of the stan- 

 dard cell always lags behind the temperature, and that due to 

 the fact that the temperature is never known exactly, are thus 

 done away with. 



(3) No assumption in regard to temperature or temperature 

 coefficient of wires is made. 



(4) Both junctions being maintained at the same temperature 

 for approximately the same length of time, and under the same 

 conditions, the likelihood of changes in physical state, produced 

 by one wire being annealed more than the other, is reduced to 

 a minimum. 



(5) The temperature is read directly by a nitrogen thermo 

 meter, and no intermediate standards need be used. 



(6) No complicated apparatus is needed, the only instrument 

 used being the galvanometer, and that only as an indicator. The 

 only standard used is the kathetometer for measuring the height 

 of the mercury column. 



The only assumption made is that the air in the bulb is at a 

 uniform temperature throughout. This assumption is justified, 

 however, by experiment. In 1890 Mr. A. E. Kennlly and the 

 writer made a number of experiments to determine the tempera- 

 ture coefficient of the electrical resistance of copper wire. In 

 these experiments the wire was wound in two coaxial coils in 

 the bulb of an air thermometer, the idea being that there would 



