REVIEWS. 33 
the water with its tail to propel itself forward, it performs a double 
task; one part consists in pushing backwards a certain mass of 
water with a certain swiftness, and the other in pushing on the 
body in spite of the resistance of the surrounding fluid. This last 
portion of the task only is utilized. It would be greater if the tail 
of the fish encountered a solid object. Almost all the propelling 
agencies employed in navigation undergo this loss of labor (travail) 
which depends on the mobility of the point d'appui. The bird is 
placed among conditions especially unfavorable. 
Professor Marey ends his first lecture with a discussion of the 
division of the muscular force between the resistance of the air 
and the mass of the body of the bird. His second and third lec- 
tures are on the resistance of the air, illustrated by mathematical 
and physical data, and the exhibition of his peculiar and delicate 
machinery for solving these problems by actual experiment. 
REVIEWS. 
Tur GEOLOGY AND PHYSICAL GEOGRAPHY or BRAZIL. *—In 
eleaning after some of the most notable of ‘the world’s travellers 
who have visited Brazil, little enough would seem to be left for 
another explorer in the same field. By steadily pursuing, however, 
for the most part one line of study, though a most comprehensive 
one, our author as a geologist has brought together in this read- 
able book a simple, clear, philosophic account of Brazilian geology 
in its widest sense, which, while doing justice to the preceding 
writers, contains a vast deal of novel information and does de- 
cided credit to American geographical and geological science. 
Our really good, carefully prepared books of travel can be 
counted on the fingers’ ends. This new candidate for favor may 
well be included among the select few. In Humboldt’s famous 
“Travels” and “ Views of Nature” we have the results of years of 
travel by a natural philosopher; in Bates’s and Wallace’s narra- 
* Thayer Expedition. Scientific Results of a Journey in Brazil. By Louis Agassiz 
and his travelling Companions. Geology and Physical Geography of Brazil. By 
Professor C. F. Hartt. With illustrations and maps. . Boston: Fields, Osgood & Co., 
1870. 8vo, i i i : 
AMER. NATURALIST, VOL. V. 3 
