INTRODUCTION. 
XXI 
through the air, must undoubtedly see better than one which 
slowly describes a waving tract. The weak-sighted bat, flying 
carefully through bars of willow, even when the eyes were ex- 
tinguished, may seem to suggest an exception to this rule of 
relative velocity and vision ; but in this case, as in that of some 
blind individuals of the human species, the e.xquisite auditory 
apparatus seems capable of supplying the defect of sight. Nor 
are the flickerings of the bat, constantly performed in a narrow 
circuit, at all to be compared to the distant and lofty soarings 
of the Eagle, or the wide wanderings of the smaller birds, who 
often annually pass and repass from the arctic circle to the 
equator. 
The idea of motion, and all the other ideas connected with 
It, such as those of relative velocities, extent of country, the 
proportional height of eminences, and of the various inequali- 
ties that prevail on the surface, are therefore more precise in 
birds, and occupy a larger share of their conceptions, than in 
the grovelling quadrupeds. Nature would seem to have pointed 
Out this superiority of vision, by the more conspicuous and 
elaborate structure of its organ ; for in birds the eye is larger in 
proportion to the bulk of the head than in quadrupeds ; it is 
also more delicate and finely fashioned, and the impressions it 
receives must consequently excite more vivid ideas. 
Another cause of difference in the instincts of birds and 
quadrupeds is the nature of the element in which they live, 
birds know better than man the degrees of resistance in the 
uir. Its temperature at different heights, its relative density, and 
uiany other particulars, probably, of which we can form no 
adequate conception. They foresee more than we, and indi- 
cate better than our weather-glasses, the changes which happen 
in that voluble fluid ; for often have they contended with the 
violence of the wind, and still oftener have they borrowed the 
a vantage of its aid. The Eagle, soaring above the clouds, can 
at will escape the scene of the storm, and in the lofty region of 
ca m, far within the aerial boundary of eternal frost,* enjoy a 
_ o mean heights of eternal frost under the equator and at the latitude of 
J and 6o° are, respectively, 15,207, 11484, and 3,818 feet. 
