Page:Science vol. 5.djvu/133

 ■BRirjun-fl, tSM.]

��117

��B wood is or Ibe big tree, yeqiioia giganlca

h.2882) ; ftnd that the lightest wood of all is


 * I fig in Florida, Ficiis aiii-ea (0.2616).

t I'poD part iii., 'The forests of the United

'Utes in their econoratc aspects,' which con-

idodea the volume, and which the fioo colored

liOiapa graphically illustrate, Professor Sargent

Itta bestowed grcnt pains, and to much purpose.

7he statistics of the lumber industry for ibe

Bitensiis _vear. the table of forest-firea during that

ear. the map showing the proportion of wood-

Uid within the settled area burned over in that

, and the map showing the character of

e fuel used in different parts of the settled

rtiou of the country, are most inteieeting

) instructive. Not less so are the detailcil

i M\y illustrated summarius of the present

niditioD and character uf the woodlands of

y state and territory.

iTlic principles of forest preservation, the

fcis of the country in this respect, and its

mrtance in certain districts, also the special

s well as great difficulty, of guarding

ainst forest-fires, are touched upon as ocea-

Em serves. If the country suffers hereafter,

I will not be from the lack of good advice.

Msibly the forest report for the eleventh ceii-

s may show that it has not all been wasted.

r the forest agent for 18D0 brings out a more

finable report than that of 18811, it will in a

peasure be due to the advantages furnished by

B work of his predecessor.

��■ This is a translation from the third German

ptioO) with large additions and notes relating

p American machines by Mr. Keith. In the

'_ t two editions of the original the work ap-

Bered in one volume ; but in the third the

r thought it desirable to divide it into two,

i in this the translators have followed him.

e first volume only is now published, and is

■ principally devoted to methods and machines

for jiroducing electric currents.

It is not easy to keep pace with the pi-oduc- tion of djTiamo-electrie 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 tbiie, for the so-called • practical' electrician.

��X*llh tB-t IVrrj ticj-n

��or for the student of electrical engineering. Dr. Schellen's book is not likely to satisfj- the demands of either of these classes.

About a hundred pages bear the general title of ' Preliminary physios.' Forty of these are occupied by the development of the funda- mental idea of the production of electricity by indnction, which is accomplished in a manner not differing greatly from that of other similar treatises. The remainder contains the con- sideiation of methods of electric measurements and measuring instruments. Including as it does dynamometric, photometric, and electric measurement prt^r, this comes near being the most unsatisfactory portion of the book. The great impoilance 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 by far the best of this part : and the translators have shown wisdom in inserting Hill descriptions of the Kent dynamometer 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 accoi'dance 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 body. ' 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 niagnclu and djniamo electric machioea in great variety. These are generally given in considerable detail, accompanied bydi^rams and plates. Many of the descriptions are very satisfactory, although raoal of them have ap- peared alrea^ly in similar publications.

The concluding chapter contains a brief discussion of the theory of dynamo -electric machines, and a classification of d3*namos. The discnasion of the theory would be greatly improveil 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. a. s. system of units of measure." lu a previous chapter the neceasily of being thorouglily fa-

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