The following letter has been on hand several weeks, but deferred on
account of a constant press of matter by which the limited space in
our former small sheet was crowded. Our respected correspondent has
consented to excuse the delay.
Providence, ---- 1846.
_Friend Porter_: In January last, I addressed a few lines to you,
asking information in regard to an article entitled Atmospheric
Resistance, in the New York Mechanic, of December 11, 1841. In your
answer, you say if the full surface is 30,000 square feet to each
wing, (which makes 60,000 square feet,) only about half of one horse
power would be required to sustain this weight, and I understand you,
virtually to say, that they must be ten times as large, in order that
the strength of one man be sufficient to work this and elevate himself
together with the apparatus, if it were not too heavy. Now, this makes
600,000 square feet. This is rather more than 774 feet square: rather
large sized wings. One would suppose that they might lift rather
heavy, if they were very light, being 387 by 774 feet each. Now, to me
this is entirely incomprehensible, and I should like an explanation,
if this calculation is correct, how it is that an eagle which
sometimes weighs nearly thirty pounds, can elevate himself, with so
much ease, and even carry with him nearly his own weight, using a pair
of wings, which if they were five feet long and two feet wide each,
would make but twenty feet of surface. Thus, you will see, is no where
in proportion to the weight even of the eagle alone, (which we will
suppose to weigh twenty pounds,) that the wings bears to the 150
pounds, while on the other hand, it is near in proportion to the
surface of the wings of a pidgeon and its weight. Nor can I comprehend
why it would require so much power, the eagle though he exerts himself
considerable in rising, no doubt, does not seem to use power any where
in the proportion that you have thought would be required supposing
the wings to be made in the same proportion to the 150 pounds that his
wings are to his weight, his beats are not so quick but what they can
be very easily counted.
By answering, you will much oblige,
your friend,
YANKEE.
In answer to the foregoing, we would remind our correspondent, that in
his former communication, he proposed a limited weight of apparatus,
and in our answer, it was far from our intention to allow an
additional weight on account of the requisite extent of surface. With
regard to the philosophy of the flight of the eagle, it must be borne
in mind that atmospheric resistance is as the square of the velocity
_downward_ and the only way in which the phenomenon of the flight
of the eagle can be reconciled with the laws of mechanical science as
established by experiment, is by supposing the velocity of the wing
downward to be equal to 70 feet per second, whereby a resistance would
be encountered equal to 12 pounds per square foot of surface to the
wings. It is a fact, however, that kites, and hawks are often seen to
continue suspended in the air several minutes without any apparent
motion of the wings; but by what law or theory the feat is
accomplished, natural philosophy has ventured no other conjecture than
that the bird is endowed with the faculty of suspending occasionally
its ordinary subjection to the laws of gravity. If any observing
theorist will give any more rational conjecture on the subject, we
should be glad to have him examine it.
* * * * *
It is proposed and urged by the papers in several States, to have a
thanksgiving day throughout the Union, on the 26th of November.
* * * * *
"As dull as a hoe," is a very common phrase, and implies that hoes are
necessarily or ordinarily dull. But it is advisible for farmers to
keep their hoes sharp, as they regard a saving of labor.
* * * * *
[Illustration: the conical windlass]