February 6, 1885.] 



SCIENCE, 



117 



ous wood is of the big tree, Sequoia gigantea 

 (0.2882) ; and that the lightest wood of all is 

 of a fig in Florida, Ficus aurea (0.2616). 



Upon part iii., 'The forests of the United 

 States in their economic aspects,' which con- 

 cludes the volume, and which the fine colored 

 maps graphically illustrate, Professor Sargent 

 has bestowed great pains, and to much purpose. 

 The statistics of the lumber industry for the 

 census year, the table of forest-fires during that 

 year, the map showing the proportion of wood- 

 land within the settled area burned over in that 

 year, and the map showing the character of 

 the fuel used in different parts of the settled 

 portion of the country, are most interesting 

 and instructive. Not less so are the detailed 

 and fully illustrated summaries of the present 

 condition and character of the woodlands of 

 every state and territory. 



The principles of forest preservation, the 

 needs of the country in this respect, and its 

 importance in certain districts, also the special 

 need, as well as great difficulty, of guarding 

 against forest-fires, are touched upon as occa- 

 sion serves. If the country suffers hereafter, 

 it will not be from the lack of good advice. 

 Possibly the forest report for the eleventh cen- 

 sus may show that it has not all been wasted. 

 If the forest agent for 1890 brings out a more 

 valuable report than that of 1880, it will in a 

 measure be due to the advantages furnished by 

 the work of his predecessor. 



SCHELLEN'S DYNAMO-ELECTRIC 

 MACHINES. 



This is a translation from the third German 

 edition, with large additions and notes relating 

 to American machines by Mr. Keith. In the 

 first two editions of the original the work ap- 

 peared in one volume ; but in the third the 

 author thought it desirable to divide it into two, 

 and in this the translators have followed him. 

 The first volume only is now published, and is 

 principally devoted to methods and machines 

 for producing electric currents. 



It is not easy to keep pace with the produc- 

 tion of dynamo-electric literature at present, 

 and one cannot avoid the conclusion that much 

 of it might be suppressed without really serious 

 loss. Books on dynamo-electric machinery 

 may be prepared for the general intelligent 

 public, for the so-called ' practical' electrician, 



Magneto and, dynamo electric machines. By Dr. H. Schel- 

 LBJT. Vol. i. Translated from the third German edition hy N". 

 8. Keith and Percy Neymann. New York, Van Nostrand, 1884. 

 0I8 p. V. 



or for the student of electrical engineering. 

 Dr. Schellen's book is not likely to satisf\' the 

 demands of either of these classes. 



About a hundred pages bear the general title 

 of ' Preliminary physics.' Forty of these are 

 occupied by the development of the funda- 

 mental idea of the production of electricity by 

 induction, which is accomplished in a manner 

 not differing greatly from that of other similar 

 treatises. The remainder contains the con- 

 sideration of methods of electric measurements 

 and measuring instruments. Including as it 

 does dynamometric, photometric, and electric 

 measurement proper, this comes near being the 

 most unsatisfactory portion of the book. The 

 great importance of thoroughly understanding 

 this part of the subject is strongly emphasized ; 

 but the reader will seek in vain for its satis- 

 factory elucidation. The study of dynamome- 

 ters is \>y far the best of this part ; and the 

 translators have shown wisdom in inserting 

 full descriptions of the Kent d3~namometer 

 prepared by Dr. Henry Morton, and of the 

 Brackett dynamometer prepared by Professor 

 Brackett, its inventor. Under electric meas- 

 urement little is to be found, aside from the 

 description of a few of the coarser devices for 

 determining electromotive force and current 

 strength, and there is really nothing concern- 

 ing methods of measurement. Although the 

 book is of very recent date, the units of 

 measure are not defined in accordance with the 

 agreement of the international electrical con- 

 gress ; and, in the discussion of photometric 

 standards, no mention whatever is made of 

 that adopted by that bod}' . ' Intensity ' for 

 current, or current strength, and ' tension ' for 

 electromotive force, are found, unfortunately, 

 throughout the work. 



The bulk of the volume is devoted to descrip- 

 tions of magneto and dynamo electric machines 

 in great variet} T . These are generally given 

 in considerable detail, accompanied by diagrams 

 and plates. Man}' of the descriptions are very 

 satisfactoiy, although most of them have ap- 

 peared already in similar publications. 



The concluding chapter contains a brief 

 discussion of the theory of dynamo-electric 

 machines, and a classification of d}~narnos. 

 The discussion of the theory would be greatly 

 improved by expansion, and the classification 

 of dynamos would be more useful to the 

 reader if introduced before the description of 

 machines. An appendix contains a number 

 of tables of considerable practical value, and 

 an attempt to define the ' absolute, or c. o. s. 

 system of units of measure.' In a previous 

 chapter the necessity of being thoroughly fa- 



