DECEMBER 22, 1899. ] 
vises his own economic profile to satisfy the 
imposed conditions. The method developed by 
tke author is an excellent one for this purpose, 
leading to no complicated equations and having 
the advantage of constantly keeping before the 
computor the statical principles of stability and 
strength. 
The fundamental assumption in these compu- 
tations is that the compressive stress on the 
base of the dam uniformly varies from a mini- 
mum value at the back face to a maximum 
value at the front or down-stream face. This 
assumption cannot be a correct one, except in 
the case of a rectangular section, but it may be 
properly used in the absence of knowledge as to 
the correct distribution of stress, because its 
errors are on the side of safety. Strictly the 
base of the dam is under a shearing stress due 
to the horizontal water pressure as well as 
under a compressive stress due to the weight of 
masonry, and of the former no account is taken 
in practical computations. Probably the error 
in the fundamental assumption regarding the 
compression more than balances the opposite 
error, due to the neglect of the shear, so that 
masonry dams designed under the common 
theory undoubtedly possess all the needed ele- 
ment of security. This conclusion may be 
justified by the fact that masonry dams rarely 
fail; the author mentions but three instances of 
failure, two of these being constructions of the 
eighteenth century when the principles of de- 
sign were not well understood, and the third 
being a case where the stone and cement were 
of so poor quality that leakage occurred. 
“Previous editions of this work were devoted 
entirely to masonry dams. ‘The present edition 
gives additional information regarding recent 
structures, and also includes the description of 
dams of earth, timber and loose rock. For such 
structures few computations are needed, the 
size and shape being determined almost entirely 
by experience, while the details may vary ac- 
cording to local conditions and the judgment of 
the engineer. The numerous devices adopted 
in California to construct rock-fill dams with- 
out leakage are of especial interest. Movable 
dams of the needle, shutter, and bear-trap types 
are also fully described; although only a few 
of these have been built in America, they will 
| SCIENCE. 931 
undoubtedly be extensively used in future river 
improvements. 
The work forms the most complete treatise 
on the subject of dams that has yet appeared. 
With commendable industry the author has 
searched the annals of engineering literature in. 
order that no important structure might escape 
notice, and his list of bibliography, covering 
five pages, will be of value to all engineers. 
From the descriptive point of view, the book 
gives nearly all needed information regarding 
the important dams of the world. From the 
theoretic point of view, it gives everything nec- 
essary regarding masonry dams which resist 
overturning by virtue of their weight alone, but 
it is somewhat lacking in regard to the theory 
of arched dams. This theory, it is true, is a 
difficult one, but, as the Bear Valley dam in 
California, and the Zola dam in France, de- 
pend for their stability largely upon the arch 
action, a numerical discussion of their stability 
would have been of interest and value. With- 
out doubt a dam arched toward the current is- 
stronger than a straight one of the same ercss- 
section, particularly in the emergency of ice 
thrust or a high flood, and it is said that the 
instinct of the beaver leads him to so construct 
them. Even if a little more material be re- 
quired, it is well for the engineer to make his- 
masonry dam an arched one and thus render 
the structure one of beauty as well as one of 
strength. 
M. MERRIMAN. 
The Botanists of Philadelphia’ and their work. 
By JoHN W. HARSHBERGER, Ph.D.  Phila- 
delphia. 1899. 8vo. Pp. 457. 
In this octavo volume of 457 pages we have a 
collection of brief biographical sketches, not 
only of all the people who have contributed to- 
a knowledge of the flora of Philadelphia and 
the area included in a radius of sixty miles, but 
nearly all who have studied it afield. Com-- 
mencing with such well-known pioneers as John 
Bartram, Humphry Marshall, Muhlenberg, Bar- 
ton, Schweinitz, and Darlington, it comes down 
to the present members of the various botanical 
clubs of the city, the whole series arranged in 
chronological order. The Bartram Gardens, the 
collections of the Philadelphia Academy of’ 
