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POPULAR SCIENCE REVIEW. 
extinct, namely, the Trilobita and the Eurypterida, which are confined to the 
Palaeozoic formations ; of three other orders, the Cladocera, the Copepoda, 
and the singular parasitic Rhizoceph ala, no fossil representatives are known ; 
hut from the Jurassic epoch onwards, all the rest of the orders appear to 
he more or less abundantly represented. Of the parasitic forms, some of 
which are among the most interesting of existing Crustacea, one can scarcely 
expect any traces to he preserved, but even of these there are indications of 
the existence of one species of Bopyrus, parasitic under the carapace of a 
crab, in the Cambridge Green-sand. Of several other groups, the number 
of known fossil representatives is but small when compared with the 
abundance of forms living at the present day, and in some cases this can 
hardly be due to the easy destructibility of the animals, which is un- 
doubtedly the cause of the disappearance of the parasites. Many hitherto 
unknown forms belonging to these groups will probably reward the labours 
of zealous searchers among the rocks ; and to all who may endeavour to fill 
up the gaps in our knowledge, either by the discovery of new forms or by 
making us better acquainted with the structure and characters of those 
numerous species which are still but imperfectly known, Mr. Woodward’s 
catalogue will be an invaluable boon. 
THE STEAM ENGINE.* 
T HOSE who wish to understand the theory and action of the steam- 
engine, but can bring to bear upon the subject scarcely more mathe- 
matics than ordinary arithmetic, will no doubt be grateful to Mr. Northcott 
for the trouble he has taken to simplify the subject. After a slender sketch 
of the history of the engine, he discusses briefly such questions as the for- 
mation of coal, the origin of force, and the nature of combustion. As to 
the thermal values of different forms of fuel, their behaviour in the furnace, 
and the best conditions for transferring heat from the furnace to the boiler, 
these are subjects so important to everyone who uses a steam-engine 
that we are glad to see them discussed with some fulness. The reader 
must not expect to find in this work a description of the constructive details 
of the various forms of steam-engine, but nevertheless the author describes 
the action of the mechanism, and compares the various forms of engine with 
respect to their efficiency. Several indicator-diagrams are given, and ex- 
plained at some length. The work also contains some useful tables, which 
have evidently cost the author much labour in their compilation. To 
render the subject intelligible to the practical engineer was Mr. Northcott’s 
principal object, but at the same time he has aimed at securing precision. 
Anyone who does not feel equal to facing such works as those of Eankine 
may dip with profit into Mr. Northcott’s volume. 
* “ The Theory and Action of the Steam Engine, for Practical Men.” 
By W. H. Northcott, C.E., &c. 8yo. London : Cassell, Petter & Galpin. 
