December i6, 1897] 



NA TURE 



149 



by any people except physiologists, and even by them 

 only in a doubtful sort of way, for they thought that its 

 indication gave merely the period and direction of a 

 sudden change of P.D., and they feared that the magni- 

 tude of the variation could not be deduced from the 

 excursion of the meniscus formed at the junction of the 

 threads of mercury and dilute sulphuric acid. 



It may be well to mention that we use the letters 

 "P.D.," not merely because they appear to us to be a 

 convenient abbreviation for " potential-difference," but 

 because Mr. Burch, by employing them throughout his 

 book, shows that he has the same opinion. 



The book commences with a description of the best 

 methods of constructing a capillary electrometer to which 

 the author's experience has led him, and the reader is 

 warned regarding the faults which he is likely to meet 

 with in the practical use of the instrument, and instructed 

 how to overcome those that can be remedied without 

 making a new instrument. 



The first two points that Mr. Burch determined to 

 ascertain experimentally, when he began his work on the 

 capillary electrometer in 1886, were : (i) Does any 

 current pass through the instrument after the meniscus 

 in the capillary tube has reached its stable position for 

 the particular P.D. applied ; and (2) does any leakage 

 take place through the instrument if the applied P.D. be 

 withdrawn after the meniscus has been deflected? To 

 each of these questions experiment gave a negative 

 answer, and he was, therefore, led to the conclusion that, 

 although the circuit of a capillary electrometer is com- 

 posed entirely of conducting substances, and, although 

 there is no visible insulating dielectric, the instrument 

 transmits no current with a P.D. of less than 0*5 volt, 

 but merely receives and maintains a charge as if it were 

 a well-insulated condenser. 



The capacity of different capillary electrometers was 

 found by the author to vary between o'l and 30 micro- 

 farads, but instruments having a capacity of between 

 o"5 and 2 microfarads gave the best results. When the 

 capacity is unchanged on moving the mercury thread 

 through a considerable portion of the capillary tube the 

 electrometer is found to be equally sensitive throughout 

 that part of the tube. 



The author concludes that this instrument is essen- 

 tially adapted to the poor man, since its cost, including 

 that of the microscope, is less than that of any other 

 electrometer of the same sensibility. It also has the 

 advantage of responding with extreme rapidity to every 

 change of potential. 



Various methods are described in detail for obtaining 

 records on photographic plates, having a rapid linear or 

 circular motion, and the author shows how the in- 

 stantaneous value of a rapidly varying P.D. can be 

 deduced at any point of the photographic curve from the 

 fact, which he proves, viz. that the value of the applied 

 P.D. is measured at any moment by the instantaneous 

 distance of the meniscus from the zero position plus the 

 rate of motion of the meniscus at that moment. And 

 in the case of the photographic plate moving circularly, 

 he points out that the second term, depending on the in- 

 stantaneous value of the velocity of the meniscus, is given 

 by the length of the subnormal to the curve at the 

 particular point. 



NO. 1468, VOL. 57] 



The book concludes with some interesting examples 

 of curves obtained with telephones, direct and alternate 

 current dynamos, &c., from which the value of the 

 capillary electrometer may be clearly seen. 



The reasoning in some parts of the book is not very 

 clear ; and, while the figures of the parts of the apparatus 

 itself are in many cases bold and well executed, those 

 illustrating the geometrical reasoning, and the results 

 obtained with the photographic plates, are not as in- 

 telligible as one would like. We hope to see Part ii. 

 of this book at an early date ; and, for the benefit of 

 those who do not resemble the author in being masters 

 in the use and the theory of the capillary electrometer, 

 we trust that he will not hesitate to sacrifice compression 

 to clearness. W. E. A. 



OUR BOOK SHELF. 



A Handbook to the Geology of Cambridgeshire, for the 

 Use of Students. By F. R. Cowper Reed, M.A., 

 F.G.S. 8vo. Pp. x2 + 76. (Cambridge : at the Uni- 

 versity Press, 1897.) 



The geology of Cambridgeshire possesses a special 

 interest for many students. From Cambridge itself 

 there have sprung a greater number of expert British 

 geologists than from any other seat of learning in this 

 country. Though founded by John Woodward, the 

 school was created by Sedgwick ; and it has been carried 

 on with signal success by Prof. Hughes. This success, 

 as the present Professor has cordially acknowledged, is 

 partly due to the band of brilliant assistants he has 

 gathered around him. Thus special instruction is given 

 in all branches of geology, and the author of the pre- 

 sent work has during recent years rendered aid in the 

 depai-tment of stratigraphical palaeontology. To a casual 

 visitor the scenery and geology of Cambridgeshire may 

 ofifer but few attractions, for the country is mostly low- 

 lying, and there is much clay-land and fen. Oxford 

 affords a greater variety of scenery and a more attractive 

 series of fossiliferous formations. Nevertheless, there is 

 much in Cambridgeshire geology to arouse interest when 

 once an enthusiasm for the science has been kindled, and 

 there was need of a concise handbook which should 

 clearly describe and explain the leading facts that have 

 been made known. The excellent "Sketch" by Prof. 

 Bonney dates back to 1875, and the more detailed geo- 

 logical survey memoir on the neighbourhood of Cam- 

 bridge, by Penning and Jukes-Browne, was issued in 

 1 88 1. The present work is a model of what a county 

 geology should be. The zones in the Jurassic and Cre- 

 taceous rocks, the phosphatic nodules in Lower Green- 

 sand and Chalk, the glacial deposits, valley-drifts, and 

 recent accumulations, are all duly described and accom- 

 panied by full records of the fossils. If we find fault at 

 all, it is that the author has entered at too great a length 

 into certain contrary views regarding the form.ation of 

 the Chalky Boulder Clay, for the extraordinary " diluvial 

 theory," as he himself admits, " finds few supporters at 

 the present time. " Even the view of S. V. Wood, jun. 

 (noted on p. 168), was modified in that geologist's latest 

 publication. 



Our author rightly wanders a little out of the county to 

 give some account of the Red Chalk of Hunstanton, be- 

 cause it is so frequently visited by Cambridge students. 



Brief chapters are given on the antiquity of man and 

 on water supply. With regard to the latter subject, we 

 would question the statement (quoted by the author) 

 that " if the Oxford Clay was pierced we might reason- 

 ably expect an abundant supply of water." The outcrop 

 of Lower Oolites is some distance away, while at St. 



