1877-] 
135 
Notices of Books. 
or gypsum, the loss of heat may, to a great extent, be prevented, 
but the pipe of course takes a longer time to heat. A convenient 
form of friction break for direCtly determining the amount of 
work done by an engine is described on p. 126, and this is fol- 
lowed by an account of means of determining the speed of the 
piston-rod, fly-wheel, and shafting, and the friction of belts and 
pulleys. The space given to the strength of materials is small, 
but this branch of the subject is always treated of at length in 
books on applied mechanics. 
Under the head of “ Meteorology ” an account is given not 
only of the determination of the more ordinary factors, but also 
of the three magnetic elements, and of the electricity of the 
air. For this latter determination Prof. Pickering proposes two 
instruments, one of which assumes the same eleCtrical potential 
as the air, while the second measures this potential. This is 
usually] effected by a Thomson’s quadrant electrometer, or a 
Peltier if less accuracy is required. To colleCt the electricity an 
insulated vessel from which water slowly drops may be used, or 
a burning match made of blotting-paper dipped in nitrate of 
lead. During the progress of a distant thunder-storm the 
electrometer shows a sudden change of potential after each flash 
of lightning. 
In the seCtion on “ Practical Astronomy ” the author discusses 
the method of using the sextant for determining latitude, longi- 
tude, and time. Also time by the transit instrument, and latitude. 
A long account is given of the mode of using the equatorial 
telescope, and, finally, of the method of spedtrum analysis 
applied to the stars. 
A short seCtion on “Lantern Projections” describes the 
various means of throwing photographs or images of apparatus 
upon a screen by means of an intense source of light, and a 
convex lens. As an apology for introducing the subject, the 
author remarks, “ During the last ten years a new era has arisen 
in the illustration of leCtures by the general introduction of the 
magic lantern as a means of demonstration. Not only in science, 
but in the mechanical arts, in architecture, and, in faCt, in any 
subjeCt susceptible of illustration by engravings or photographs, 
a few glass plates which may be carried in the hand will interest 
and instruct an audience more than the finest diagrams, which 
are, moreover, far more cumbrous and expensive. It is, therefore, 
desirable that every one who may have occasion to address an 
audience should be able to manage a lantern, and to project 
photographs on the screen. Again, especially in physical 
experiments, many objects are so minute that they cannot well 
be shown to a large number of persons, and an enlarged image 
of these may often be thrown on the screen, and thus be seen 
by hundreds at a time.” The best methods of using sunlight, 
the eleCtric light, the magnesium light, and the oxyhydrogen, or 
calcium light (as the author calls it) are described. For a 
