438 Notices respecting New Boohs. 



The one set of messages is to be transmitted and received by an 

 ordinary Morse installation working with constant currents. For 

 the other set of messages the phonopore is used, its primary coil 

 being connected simply to a spring by which its current is 

 automatically made or broken periodically, and to a key for sending 

 messages. The secondary coil has one end insulated and the other 

 connected to the line. The second set of messages is therefore a 

 series of breaks in an otherwise periodically made and broken 

 current. The receiver for these messages is somewhat complicated 

 and very ingenious. It possesses a spring whose rate of vibration 

 coincides with that of the harmonically interrupted current, and 

 it is therefore not affected by irregular disturbances. The system 

 seems quite good in principle, and is an ingenious and extremely 

 interesting attempt to put the idea of Elisha Gray's harmonic 

 telegraph into a workable form. The author points out that his 

 telegraph can be added to any previously existing installation, and 

 he has made crucial experiments on several lines both in England 

 and abroad. The reports given by eye-witnesses of these experi- 

 ments are incorporated in the book before us, and they seem to 

 promise well for the ultimate success of the system. J. L. H. 



Reduction Tables for Readings by the Gauss-Poggendorff Mirror 

 Method. Czermak. (Berlin : Julius Springer.) 



Whenever a mirror-and-scale method is used for the measure- 

 ment of small angles some trouble is experienced in reducing the 

 observations, because the scale-readings are proportional to the 

 tangent of twice the angle of deflexion, whereas we generally re- 

 quire either the angle of deflexion itself, its tangent, or else, as 

 in the ballistic galvanometer, the sine of half of it. The object of 

 these tables is to enable an observer to write down at once, know- 

 ing the distance between his mirror and scale and the scale- 

 reading, a number proportional to the quantity he wishes to 

 measure in any experiment. There are four tables given in order 

 to find the angle of deflexion in degrees or in circular measure, 

 the tangent of the angle of deflexion and the sine of half of it, 

 respectively. The method of reduction and the process by which 

 it is arrived at are described in English, Erench, and German, at 

 the beginning of the book. The tables will prove useful to those 

 who are in the habit of making series of observations with a mirror 

 and scale, though the method is far more frequently resorted to 

 on the Continent than in England, null methods being here in more 

 general use. J. L. H. 



An Introduction to Dynamics, including Kinematics, Kinetics, and 

 /Statics, with numerous examples. By C. Y. Burtojst, D.Sc. 

 (London: Longmans. Pp. xiii + 392.) 



The first seven chapters (with the exception of chapter in. which 

 treats of the trigonometry of one angle, for the benefit of readers 



