1877.] 
Notices of Books. 
137 
If the catapult is used at all, attach it to the table, and determine 
the effeCt of drawing the spring by known amounts, and also by 
varying the weight of the ball.” 
“ 241. Devise a form of photometer for measuring the amount 
of light reflected at various angles by polished surfaces, in which 
a single light only shall be used.” 
“ 292. Measure the density of fog, or the amount of light ab- 
sorbed by layers of various thicknesses.” 
At a time like the present, when physical laboratories are 
making their appearance at our universities, and even at some 
of our public schools, Prof. Pickering’s book is a useful contri- 
bution to Science. It seems to us, however, to be far too meagre, 
and its value would be greatly enhanced by the introduction of 
really good woodcuts of the nature of those to be found in 
Deschanel. There can be no doubt that the volume will grow 
with each succeeding edition, and many improvements may be 
then introduced ; meanwhile we recommend it to students who 
work in a physical laboratory as a really useful book, where they 
can apply for information concerning any part that is obscure 
to the professor in charge of the laboratory. 
