482 



NA TURE 



[June 



=o> 



»9io 



of the upper surface of Coletis (or Teracolus) atnata 

 is well described as "orange." "Thaxter" on p. 405 

 is a misprint for Thayer. These, and a few similar 

 slips, are but slight blemishes on a thoroughly useful 

 book. F. A. D. 



THEORETICAL STUDIES IN RELATION TO 

 NAUTICAL SURVEYING. 

 Hydrographic Surveying: Elementary — for Beginners: 

 Seamen and Others. A Practical Handbook. By 

 Commander Stuart V. S. C. Messum. Pp. xiv + 504. 

 (London : C. Griffin and Co., Ltd., 19 10.) Price 12s. 

 net. 



T^HEORETICAL considerations dealing with the 

 J- effect of errors of observation constitute one of 

 the distinctive features of this book. It may be 

 doubted whether such investigations are appropriate to 

 a practical handbook intended for beginners; and in 

 some instances, where they are merely of academic 

 interest, their introduction is unnecessary. The dis- 

 sertations on the manipulation of the station pointer, 

 for example, are diffuse and of little practical utility. 

 The chapter on the principles governing the selection 

 of objects for fixing positions contains certain 

 theorems which will be of interest to those already 

 familiar with the subject; but beginners would find 

 concise directions as to what goes to make a good fix 

 more helpful to them. The discussions bearing on 

 the "circle of equivalents" bring out a useful fact in a 

 form somewhat different from that in which it has 

 usually been presented. The principle involved is an 

 important one, but it is possible to strain unduly its 

 practical application. So much stress has been laid 

 on it that beginners might easily be misled ; the author 

 himself appears to have misapplied the principle on 

 more than one occasion. 



This is notable in the example of plotting given on 

 p. 197, where it is suggested to accept an 

 intersection of two lines cutting each other 

 at an angle of a little more than 30°, 

 in preference to an intersection of about 90°. 

 In this case primary points are alone concerned, and 

 the considerations indicated by the author are not 

 applicable. A similar misapprehension is noticeable 

 on p. 201, in the paragraph relating to the projection 

 of check lines from the best lines of reference. It is 

 here implied that in the case of primary points, one of 

 the lines on which they are plotted might have been 

 laid off with a length of radius so short as to vitiate 

 any lines laid off from it subsequently. 



The question is discussed at some length as to 

 the best zero to select for shooting up other objects 

 when the position of the observer is not accurately 

 determined. The problem is one of frequent occur- 

 rence, and is of great importance, but the treatment 

 it receives is not satisfactory, and is liable to mis- 

 apprehension. In this, as in some other cases, the 

 broad practical rule of choosing a zero situated at about 

 the same distance as the object to be shot up and 

 making as small an angle with it as possible is not 

 stated; whilst the investigation rests on assumptions 

 not realisable when drifting in a boat or the ship in 

 NO. 2121, VOL. 63 J 



an unknown direction, as always happens in practice 

 In the paragraph on measuring a base by chainec 

 portions, the rule of sines is used for solving triangle; 

 having one very obtuse angle and two acute angles 

 The proper method of solving such triangles migh 

 have suggested to the author the fact that since cosine 

 of small angles change very slowly, small errors o 

 observation are practically of no account, and conse 

 quently that the measurement of the off-set is un 

 necessary and less accurate than using each separat 

 section of the base as measured. 



Other instances of misapplication of theory to prac 

 tice might be quoted, but those mentioned suffice ti 

 indicate a want of appreciation of practical require 

 ments, and suggest the possibility that the author i 

 more familiar with the theoretical study of the subjec 

 than with the conduct of a survey. 



In discussing the question of false station, there i 

 no reference to the simple method of elirhinating a. 

 errors from that source by the expedient of observin; 

 at equal distances on opposite sides of the tru 

 station. There is, moreover, an easier method tha 

 that given by the author for calculating the correctio 

 for false station. 



The use that might be made of a distant peak i 

 connection with the angle of elevation of the masj 

 head when sounding a shoal has also escaped atter 

 tion ; neither is there any reference to the use 

 angles of elevation in making a running survey 

 an island when circumstances admit. 



The investigation of the error of parallax in coi 

 nection with sextant angles, due to the use of the Ion 

 telescope when reflecting objects close to the observe 

 is of some theoretical interest, and is worthy of mei 

 tion. As a matter of fact, the error from this soun 

 is not nearly so great as the author assumes, sin< 

 it only exists in a minor degree with the short tel 

 scope which in practice is always used, being moi 

 convenient. 



The various instruments and the methods of usin 

 ' them are fully described, and the ordinary operatioi 

 connected with surveys of small extent are given 

 detail, together with a number of examples of sue 

 surveys, besides a useful chapter on amending tj 

 details of a chart, A. M. F. 



PRODUCTION OF SEED-OILS. 



Linseed Oil and other Seed Oils : An Industri 

 Manual. By Prof. W. D Ennis. Pp. xiv + 3i 

 (London : Constable and Co., Ltd., 1909.) Prft 

 i6s. net. 



IN this work the author aims at the production 

 a manual which will serve as a fairly complete gui 

 for the manufacturer of certain seed oils, more 

 ticularly linseed oil. He notes that, with one or t^ 

 exceptions, the principal publications dealing wl 

 this subject hitherto have discussed it chiefly fnp 

 the chemical standpoint. Accordingly in this voluiJ 

 the chemistry — which, after all, is relatively simple 

 is subordinated to the manufacturing and commerd 

 aspects of the industry. 



The work is written from the American point 



