VAST 9, I8W.I
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��permanently closed, or are proleLtod by
ant^ebambers; and those of someiiiarsh-iilaiita CHnnot close at all. In sunny places the iiii'in llie iDtercelliilui' spaces is in motion, nnil may he oliserved passing out by the stomatn. Dur- ing the lire of the plant, two mnximii ol' trans- piration occur: (1) in 3-outb, the air passing Uirough the soft cuticle; (S) In adult life, when ' jiasses by the stomata.
""lie suggestion of Saclis, that the narroiv- of the cells of autumn wood of trees re- from t«nBion, is unsatisfactory, beciuise fie change from broad to narrow cells is sud- den, and the tension upon the young wood is nearly the same in antumn as in spring. How the difference is c.iused is not known; luit it benefits tlio tree by affording wide channels for a plentiful supply of water for the opening leaves of spring anrt for the excessive Irans- ktion of summer, ami. on the other hand, by iding thickness and strength to meet the las of winter. G. MAi.r.osKiK.
��TROWBRIDGE'S PHYSICS. who are Interested in the improvement \ elementary science- teaching must reganl 1th DO little interest the announcement that u If sidst of Professor Trowbridge's deservedly ~li reputation and great experience has taken e to prepare a text-book in physics for set- tdary schools. ' The new physics ' is cer- [nly not of the common type of text-books, irri it will be generally welcomed as, in many aspects, a new departure,
li^xercises in measurement occur from the legioning, and the student is shown the im- portance of ' (Inding out things for himself at "^X early stage. The book is rich in sugges- His concerning the construction and use of psple forms of apparatus, by means of which "Tortant physical constants rna3- be deter- ^ with some precision. For linear meas- lent such instruments as the vernier, tlu- Urometer. the cathetonieter, and the micro- _ _ e with cobweb micrometer eye-piece, which |i often among the more expensive appliances \ a physical laboratory, are described, and nr construction so planned as to tempt any lerprising high-school teacher to undertake 'r manuiiicture. .'Several ingenious methods EmeasDring small intervals of time are intro- ', and most of them are so simple that ^r value can be tested at little expense. Ity Mus of these methods the laws of motion i investigated experimentally: in fact, the
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��attempt is made to diseovcr wh:il these laws are, and not merely to verify them. The stu- dent is taught how to construct galvanometers and electrometers, and how to use them in elec- trical measurement. In ahorl, what may be termed the laboratory metho<l of teaching ele- mentary physics is adopte<l bj' the author with- out reserve.
But it is a great disap{X>intment to Bnd a book containing so much that is fresh and original so marred by errors, many of which are i-eally serious. The laudable attempt has been made to put the student in possession of certain principles of prime importance which are generally to be found only in the collf^e text-book, and not always there. In the dis- cussion of some of these, mistakes of consid- erable magnitude, and statements that are very misleading, have unfortunately found their way into the text. Of these, some of the most serious are to be found in the chapter un mo- ments of luertla. In attempting to calculate, without involving the element of time, the force with which a steel spring strikes a pen- dulum ball, some inconsistent and extraonli- nary equations are produced. A little ftjrther on the reader will be astonished to Dnd it de- monstrated (?) that in a lever the products of each force by the square of its distance from the fulcrum are equal: and on this proposition the principle of moments of inertia is allowed to rest. The statement is also made that the radius of gyration is the length of the equiva- lent simple pendulum; and this error permeates the whole treatment of simple and comixiund pendulums. In the dcHnition and discussion of equipotential surfaces the false assumption is made that force is constant over such a sur- face. Preliminary to the consideration of the work done by an electric cunent will be found a brief discus.<iion of the dimensions of foi-ce and work, which is obscure and misleading.
There will be considerable difference of opinion about the propriety of inserting in an elementary test-book such matter as the <le- termination of the value of the ohm in absolute unite, the measure of the horizontal com|H>- nent of the earth's magnetism, and the meas- ure of electromotive force by the " throw ' of a galvanometer- needle.
The book is extremely suggestive, and will be found of great use iu the hands of the enthusiastic teacher. A second edition will doubtless be free from the numerous mistakes of the present, which can hardly be regarded as a safe guide to one not alreadj- tolerably familiar with the underlying principles of the ' new physics.'
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