April 20, 1905J 



NA rURE 



579 



A clear account is given in the initial paragraphs 

 of the method of observations and the instruments 

 used, but not too much space is devoted to these 

 details, so that the reader is soon brought to the 

 first difficulty which occurred in the working out of 

 the observations. It was necessary, in order to reduce 

 them to a common epoch, to take account of secular 

 variations. This might most easily have been done 

 by choosing as observing stations the same places 

 at which the magnetic elements had been determined 

 in a previous survey, but in attempting to carry this 

 out it was found that the changes which had taken 

 place in their surroundings made it impracticable 

 to observe at most of the old stations. Some 

 other method of reduction had therefore to be 

 adopted. Empirical expressions were found for the 

 magnetic elements in terms of longitude and latitude 

 similar to those deduced by Prof. Knott for the 

 previous survey. K comparison of the two ex- 

 pressions gave the secular variation. The results of 

 all the observations for each station are given in 

 the report. The reduction of the observations to 

 sea level is always to some extent arbitrary. The 

 process employed in the present case, where use is 

 made of relations given by the theory of the potential 

 between the radial variation of the horizontal com- 

 ponents and the horizontal variation of vertical force, 

 is an improvement on the more empirical methods 

 which have sometimes been adopted. 



A further application of the potential theory may 

 serve as an important check on the accuracy of the 

 observations. If a potential exists, the rate of vari- 

 ation of the northerly force towards the west must be 

 equal to the rate of variation of the westerly force 

 towards the north. If this relation does not hold, the 

 earth's magnetism cannot be completely represented 

 by a potential, and this would mean that vertical 

 electric currents traverse the earth's surface. The 

 authors of the present survey calculate the intensities 

 of these vertical currents, but rightly do not attach 

 much importance to them. They are much greater 

 than observations on atmospheric electricity allow us 

 to contemplate as possible. We may therefore take 

 the calculated values of these currents to be indica- 

 tions of the extent of uncertainty in the observations. 



We must refer the reader to the original for the 

 discussion of local disturbances, but cannot avoid 

 directing attention to one passage, which seems to 

 indicate some kind of misapprehension on the part 

 of the author. 



" It is often erroneously believed," he says, "that 

 the expansibility of the earth's magnetic potential in 

 negative powers of the radius vector is a proof that 

 the source of action is inside the earth." 



In a preceding sentence the writer connects his 

 supposed error with the fact that " inasmuch as the 

 surface integral of the force over the earth vanishes, 

 the so-called seat of action may be placed either inside 

 or outside." 



In this passage the author seems to doubt a well- 

 established theorem which is quite independent of the 

 question whether the surface integral of normal force 

 when taken over the whole surface of the earth has 

 a finite value or not. 



NO. 185 1, VOL. 71] 



To put the matter plainly : If the magnetic forces 

 at all points of the surface of a sphere can be repre- 

 sented in terms of a potential which is expressed as 

 a series of spherical harmonics proceeding by negative 

 powers of the radius vector, then there are no mag- 

 nets or electric currents outside the sphere. If the 

 passage quoted is intended to deny the truth of this 

 proposition, the author is guilty of a heresy which 

 he does not justify either by his hydrokinetic analogy 

 or by his reference to one of Lord Kelvin's papers. 

 It should be said, however, that in other parts of his 

 volume the author seems to adopt Gauss's reasoning 

 as to the discrimination between outside and inside 

 effects by spherical harmonic analysis. It may be, 

 therefore, that the apparent meaning of the passage 

 is not the one which it was intended to convey. It 

 is of some importance to avoid misunderstanding on 

 so important a matter, and it is for this reason that 

 I feel compelled to direct attention to the only 

 criticism which can fairly be raised with regard to 

 a very meritorious and heavy piece of work. 



May other countries follow this example of 

 Japanese enterprise, and may, especially in English 

 colonies, scientific men receive such help from their 

 Governments as will enable them to keep pace with 

 foreign nations in the successful prosecution of 

 similar work. It is not the enterprise or the know- 

 ledge which is wanting, but the material assistance and 

 the official recognition that a certain duty is imposed 

 on each country to take its share in the working out 

 of geophysical problems. Arthur Schuster. 



THE TECHNOLOGY OF THE VEGETABLE 

 FIBRES. 



The Spi)ining and Twisting of Long Vegetable Fibres 

 (Flax, Hemp, Jute, Tow, and Ramie). By Herbert 

 R. Carter. Pp. xvi + 360. (London : Chas. Griffin 

 and Co., Ltd., 1904.) Price io5. net. 

 ■\170RKS written for the textile industries may be 

 divided into three classes, viz. descriptive works 

 of a more or less technical and practical character, 

 educational works leading students up to an appreci- 

 ation of the difficulties to be faced, and works which 

 combine the descriptive and educational but which 

 too frequently meet the requirements of neither 

 manager nor student. The work under consideration 

 meets the requirements of the mill manager or 

 advanced student in a manner perhaps more than 

 satisfactory. On the other hand, to place such a 

 work as this in the hands of the elementary student 

 would be anything but satisfactory, rather suppress- 

 ing than (developing that genuine interest without 

 which it is impossible for the student to make true 

 progress in his studies. In its particular line, how- 

 ever, we must highly commend the work as repre- 

 senting up-to-date practice in most of the sections of 

 the textile industries of which it treats. 



The work is really arranged in four sections, the 

 first three chapters being devoted to general par- 

 ticulars respecting the fibres in question, chapters iv. 

 to XV. dealing with the mechanical processes necessary 

 for the formation of the said materials into satis- 

 factory yarns, chapters xvi. and xvii. referring to 



