460 



NA TURE 



[March 17, 1898 



In a book of this description it would be invidious 

 to criticise too closely, or we might be tempted to ques- 

 tion some of the definitions of terms in the opening 

 chapter, such as "Farad — the practical unit of capacity" 

 and " Shunt — a conductor, usually a resistance box, for 

 leading into another channel part of a current that is 

 too powerful for the immediate purpose." 



The idea, however, seems to have been to convey to 

 the reader the practical sense of things, and not to split 

 hairs as to choice of terms. The chapter on that elec- 

 trical pons asinorum, the theory of the Wheatstone 

 Bridge, is especially clear and well put, and the student 

 who reads and digests it thoroughly may cease to frown 

 at the name of Kirchhoff, the sum of whose quantities 

 has frequently an aggravating habit to the student 

 mind, of equalling nothing. 



Paragraphs 54-55, pp. 55-59, explaining the necessity 

 for ascertaining the proper time constants to be allowed 

 in charg ing long cables, whose resistances it is required 

 to measure, are very important to learners, and should 

 be carefully studied. Three simple rules are given for 

 determining these constants in the cases of (i) a perfect 

 cable, (2) a broken cable, and (3) a faulty cable, and it is 

 suggested that an ordinary metronome, such as is used 

 in beating time for music, will be found of great assist- 

 ance in measunngthe time for charging and discharging. 

 Articles have recently appeared in the technical 

 journals, and some discussion has followed, upon the 

 relative merits of the " scale zero " as opposed to the 

 " false zero " method, for fault localisation especially. 

 Upon this head the authors remark : " We will now 

 describe the ' false zero ' method. We consider this 

 method by far the best and most accurate . .. . especially 

 with a faulty cable " ; and again at p. 54, with special 

 reference to Mance's method for eliminating the resist- 

 ance effect of an earth current, they observe : " It may 

 be pointed out here that the statement that R, the re- 

 sistance equivalent to the E.C. varies inversely as c, the 

 current to line, is not quite correct in the case of faulty 

 cables. If the testing current be reduced or increased, it 

 slightly reduces or increases the E.M.F. due to polarisa- 

 tion of the fault, and therefore the current set up 

 thereby, but we have not found this error to be as a 

 rule appreciable." 



The tests for the measurement of capacity are clearly 

 explained as far as they go, and numerous references are 

 given to various text-books and other publications, to 

 enable the student to follow out for himself the study of 

 the effect of the phenomenon of absorption upon different 

 dielectrics. 



In the second part of the book, a description of the 

 various tests employed for localising partial faults, and 

 total breaks in submarine cables, is prefaced by a few 

 interesting preliminary remarks upon the nature of these 

 faults, and their behaviour under the influence of the 

 testing current. I 



It is interesting also, and instructive, to note the j 

 headings under which they are grouped, such as those 

 due to submarine borers, to chemical 2,c\\QXi, fish-bites, 

 punctures, &c. 



We must, however, take exception to the expression 

 " Kennelly's law " in the description of his tests for 

 total breaks. A "law" in this sense must be absolute 



and unchangeable, whereas Kennelly's '^rule" that 

 "when the exposed area of a break s constant, its re- 

 sistance varies inversely as the square root of the current 

 strengths passing through it " is only true between certain 

 limits of current. Kennelly pointed this out himself, and 

 Schaefer has since demonstrated that beyond 25, and up 

 to 50 mini -amperes, the resistance varies inversely as the 

 r3th root of the current strengths. 



The authors have endeavoured to make the various 

 sections of their book complete in themselves, and have 

 added a number of diagrams of connections for signalling 

 on cables, which form a new feature in works of this 

 description, and which, with the various tables of 

 temperature coefficients, will no doubt be found useful. 



We may add that nearly all the examples are actual 

 tests of cables, and are given very fully in the hope that 

 they may tend to throw a practical light upon the ex- 

 planations of the various tests. 



In conclusion, we feel it incumbent upon us to re- 

 commend every student of submarine cable engineering 

 to add this little volume to his library. 



BRITISH MOTHS, AND THE GENUS 

 DIANTHCECIA. 



The Lepidoptera of the British Islands. A Descriptive 

 Account of the Families, Genera and Species indigenous 

 to Great Britain and Ireland, their Preparatory States, 

 Habits and Localities. By Charles G. Barrett, F.E.S. 

 Vol. iv. Heterocera. Noctuae. 8vo, pp. 402. (London : 

 Reeve, 1897.) 



THE fourth volume of Mr. Barrett's elaborate work 

 includes ninety-eight species (three of which, how- 

 ever, are regarded by the author as doubtfully British) 

 distributed among thirty genera of the Noctuce Trifidce, to 

 which group belong by far the larger part of the European 

 stout-bodied, night-flying moths. This gives us an average 

 of something like four pages to each species, though 

 many of the notices run to six or seven pages ; and op 

 such a scale it will require twenty volumes to complete 

 the life-histories of our British Lepidoptera. 



Mr. Barrett is well known as one of the best of our 

 practical entomologists, and the history of every species 

 is worked out with extreme care, the observations of 

 numerous lepidopterists throughout the British Isles 

 being freely utilised, in addition to those of the author 

 himself. The foreign range of each species is also briefly 

 indicated, though the character and extent of the book 

 prevents almost any allusion to allied species not occurring 

 in Britain, except so far as such notices are absolutely 

 necessary to elucidate British species. 



Considerable space is devoted to the genus Dianthoecia, 

 which includes eight or nine British species, most of which 

 are coast-frequenting insects with us, the moths being 

 found flying over Silene in the evening, often on precipitous 

 sea-chfTs, where it is not unfrequently a task of some 

 difficulty and danger to capture them. The larvae are 

 described by Mr. Barrett as " smooth, plump, with the 

 head small, usually feeding on or in the blossoms or 

 seed-vessels of plants belonging to the Caryophyllacece." 



Britain is called by the French entomologists, le pays 

 de varices; and the British species of Dianthcecia are 



NO. 1481, VOL. 57] 



