952 ON THE FLIGHT OF BIRDS. 
they will sail round and round very slowly, six, eight, even ten 
times, in the course of five to ten minutes, without even once 
moving their wings, only slightly inclining the tail in varying 
directions so as torise and fall a little, as they revolve, 
their rounds being not circles, but a series of ascending and de- 
scending spirals. 
Now what keeps the Vultures up, and @ fortiori, what keeps 
these Crows up ? Stuff a Crow carefully, as I have done, with 
wings and tail extended; let your stuffed Crow, like mine, 
weigh 5ozs. instead of llb. 80zs. which the live Crow weighs ; 
get a multiplying wheel with a thin silk twine, 30 yards long, 
weighing an ounce at most; attach it to the skin so that this will 
sail straight without twisting ; place the skin on the top of a post 
15 or 20 feet high so as to give it a good start; then whirr the 
wheel, and your dead Crow shell will come through the air 
to you three times as fast as ever the live Crow succeed- 
ed in making his way, but.........the skin will have hit the 
ground before reaching you. Yet the live Crow, weighing 
more than four times what the skin does, circles round and 
round my head without one single action that could, with refer- 
ence to resistance of the air, &., account for its not falling, at 
certainly less than one-tenth of the velocity which I, by 
mechanism, impart to the skin. 
I venture to hope that no one will fatuously revive that old 
exploded fallacy of air-cells filled with heated air. If all 
the cells were filled in the case of a Crow with air at 
a temperature of 160° F., the outside air being at 70° F., 
the raising power thus engendered would not suffice to sustain 
a single ounce weight, whereas my skin that won’t keep up, 
weighs only 540zs. against llb. 80zs. of the live bird. 
But for all that the live birds do keep up, and there has 
never yet been, so far as I am aware, any explanation of their 
so doing, that can, when tested, be accepted. 
The real explanation is simple enough, but I do not doubt 
that when I set it forth, especially when I explain, as I must, 
how I was led to suspect it, my statement will be received 
much as the Crows used to receive Brooks’ walking-stick 
demonstrations. 
The only difference will be, that, whereas the Crows, having 
only instinct to guide them, were right in the view they took 
of the case ; the intellectual people, who will reject my explana- 
tion, will be wrong. Well, they will know better some day. 
Every great truth is a folly to the generation in which it first 
shows itself. 
Now did any of my readers ever hear of £throbacy? Of 
course the majority reply: “Is it anything to drink? Is it 
good ?” No, it is not a potable article; it is a fantastic name, 
