KEVIEWS. 
415 
with the stages of insect development, and thus we have four different sec- 
tions in the work, corresponding respectively to the egg, larva, chrysalis, 
and imago. Under these heads there is nothing left untold which could he 
said ; and as Dr. Knaggs is not a compiler, but simply an entomologist who 
writes his experiences, everything he says is worthy of attention. In this 
little volume the butterfly hunter will find all his difficulties smoothed over ; 
and if he wishes to know how, where, when to observe, capture, rear, hatch, 
preserve, and feed insects, he can have no better guide than Dr. Knaggs. 
IRON BRIDGES.* 
T he feature which characterises the architecture of the present age is the 
extensive employment of iron in nearly all structures. It has com- 
pletely taken the place of timber, and with the best results, and in many 
cases is used as a substitute for stone and brick. But the use of iron has in- 
volved the necessity for skilled calculation of its strength, its power to resist 
strains, its elasticity, and so forth, such as the older architects never dreamed 
of. It is of the utmost moment that these points should be thoroughly 
understood, and it is satisfactory to find that so excellent an authority as 
Mr. Unwin has published a clear and tolerably simple book for the use of 
mechanical engineers who have to deal with iron structures. It is one of the 
characters of this work that it explains the graphic method — now so general 
in other branches of science — for the determination of data for estimating 
the pressure and oscillation of iron structures, such as bridges. 
SCIENTIFIC AGRICULTURE.! 
T he first volume of this work, published a couple of years ago, was chiefly 
if not wholly written by the principal of the Cirencester Agricultural 
College. The present volume comprises a series of essays, some scientific 
and some politico-economical, and by various writers. Among the more im- 
portant chapters are those on experiments on wheat, grass, and barley, by 
Professor Wrightson,on the absorptive powers of soils by Mr. R. Warington, 
and on the distribution of tribasic phosphate of lim^, by Professor Thistleton 
Dyer. 
* « Wrought-iron Bridges and Roofs. Lectures delivered at the Royal 
Engineer Establishment, Chatham.” By W. Cawthorne Unwin, B. Sc. 
London : Spon, 1869. 
t Practice with Science,” vol. ii. Longmans. 1869. 
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