REPLY FROM THE OBSERVATORY OF CAMBRIDGE.
Barbicane, however, lost not one moment amidst all the enthusiasm of which
he had become the object. His first care was to reassemble his colleagues
in the board-room of the Gun Club. There, after some discussion, it was
agreed to consult the astronomers regarding the astronomical part of the
enterprize. Their reply once ascertained, they could then discuss the
mechanical means, and nothing should be wanting to ensure the success of
this great experiment.
A note couched in precise terms, containing special interrogatories,
was then drawn up and addressed to the Observatory of Cambridge in
Massachusetts. This city, where the first University of the United States
was founded, is justly celebrated for its astronomical staff. There are
to be found assembled all the most eminent men of science. Here is to
be seen at work that powerful telescope which enabled Bond to resolve
the nebula of Andromeda, and Clarke to discover the satellite of Sirius.
This celebrated institution fully justified on all points the confidence
reposed in it by the Gun Club.
So, after two days, the reply so impatiently awaited was placed in the
hands of President Barbicane.
It was couched in the following terms:—
"_The Director of the Cambridge Observatory to the President of the Gun
Club at Baltimore._
"CAMBRIDGE, _Oct._ 7.
"On the receipt of your favour of the 6th inst., addressed to the
Observatory of Cambridge in the name of the Members of the Baltimore
Gun Club, our staff was immediately called together, and it was judged
expedient to reply as follows:—
"The questions which have been proposed to it are these,—
"'1. Is it possible to transmit a projectile up to the moon?
"'2. What is the exact distance which separates the earth from its
satellite?
"'3. What will be the period of transit of the projectile when endowed
with sufficient initial velocity? and, consequently, at what moment ought
it to be discharged in order that it may touch the moon at a particular
point?
"'4. At what precise moment will the moon present herself in the most
favourable position to be reached by the projectile?
"'5. What point in the heavens ought the cannon to be aimed at which is
intended to discharge the projectile?
"'6. What place will the moon occupy in the heavens at the moment of the
projectile's departure?'
"Regarding the _first_ question, 'Is it possible to transmit a projectile
up to the moon?'
"_Answer_.—Yes; provided it possess an initial velocity of 1200 yards
per second; calculations prove that to be sufficient. In proportion as
we recede from the earth the action of gravitation diminishes in the
inverse ratio of the square of the distance; that is to say, _at three
times a given distance the action is nine times less._ Consequently, the
weight of a shot will decrease, and will become reduced to zero at the
instant that the attraction of the moon exactly counterpoises that of
the earth; that is to say, at 47/52 of its passage. At that instant the
projectile will have no weight whatever; and, if it passes that point,
it will fall into the moon by the sole effect of the lunar attraction.
The _theoretical possibility_ of the experiment is therefore absolutely
demonstrated; its success must depend upon the power of the engine
employed.
"As to the _second question_, 'What is the exact distance which separates
the earth from its satellite?'
"_Answer._—The moon does not describe a circle round the earth, but
rather an _ellipse_, of which our earth occupies one of the _foci;_ the
consequence, therefore, is, that at certain times it approaches nearer
to, and at others it recedes farther from, the earth; in astronomical
language, it is at one time in _apogee_, at another in _perigee._ Now the
difference between its greatest and its least distance is too considerable
to be left out of consideration. In point of fact, in its apogee the moon
is 247,552 miles, and in its perigee, 218,657 miles only distant; a fact
which makes a difference of 28,895 miles, or more than one ninth of the
entire distance. The perigee distance, therefore, is that which ought to
serve as the basis of all calculations.
"To the _third_ question:—
"_Answer._—If the shot should preserve continuously its initial velocity
of 12,000 yards per second, it would require little more than nine
hours to reach its destination; but, inasmuch as that initial velocity
will be continually decreasing, it results that, taking everything into
consideration, it will occupy 300,000 seconds, that is 83hrs. 20m. in
reaching the point where the attraction of the earth and moon will be
_in equilibrio._ From this point it will fall into the moon in 50,000
seconds, or 13hrs. 53m. 20sec. It will be desirable, therefore, to
discharge it 97hrs. 13m. 20sec. before the arrival of the moon at the
point aimed at.
"Regarding question _four_, 'At what precise moment will the moon present
herself in the most favourable position, &c.?'
"_Answer_.—After what has been said above, it will be necessary, first
of all, to choose the period when the moon will be in perigee, and also
the moment when she will be crossing the zenith, which latter event will
further diminish the entire distance by a length equal to the radius of
the earth, i.e. 3919 miles; the result of which will be that the final
passage remaining to be accomplished will be 214,976 miles. But although
the moon passes her perigee every month, she does not reach the zenith
always at _exactly the same moment._ She does not appear under these two
conditions simultaneously, except at long intervals of time. It will be
necessary, therefore, to wait for the moment when her passage in perigee
shall coincide with that in the zenith. Now, by a fortunate circumstance,
on the 4th December in the ensuing year the moon _will_ present these two
conditions. At midnight she will be in perigee, that is, at her shortest
distance from the earth, and at the same moment she will be crossing the
zenith.
"On the _fifth_ question, 'At what point in the heavens ought the cannon
to be aimed?'
"_Answer_.—The preceding remarks being admitted, the cannon ought to
be pointed to the zenith of the place. Its fire, therefore, will be
perpendicular to the plane of the horizon; and the projectile will soonest
pass beyond the range of the terrestrial attraction. But, in order that
the moon should reach the zenith of a given place, it is necessary that
the place should not exceed in latitude the declination of the luminary;
in other words, it must be comprised within the degrees 0° and 28° of
lat. N. or S. In every other spot the fire must necessarily be oblique,
which would seriously militate against the success of the experiment.
"As to the _sixth_ question, 'What place will the moon occupy in the
heavens at the moment of the projectile's departure?'
"_Answer_.—At the moment when the projectile shall be discharged
into space, the moon, which travels daily forward 13° 10' 35", will be
distant from the zenith point by four times that quantity, i.e. by 52°
42' 20", a space which corresponds to the path which she will describe
during the entire journey of the projectile. But, inasmuch as it is
equally necessary to take into account the deviation which the rotary
motion of the earth will impart to the shot, and as the shot cannot reach
the moon until after a deviation equal to 16 radii of the earth, which,
calculated upon the moon's orbit, are equal to about eleven degrees, it
becomes necessary to add these eleven degrees to those which express the
retardation of the moon just mentioned: that is to say, in round numbers,
about 64 degrees. Consequently, at the moment of firing the visual radius
applied to the moon will describe, with the vertical line of the place,
an angle of sixty-four degrees.
"These are our answers to the questions proposed to the Observatory of
Cambridge by the members of the Gun Club:—
"To sum up,—
"1st. The cannon ought to be planted in a country situated between
between 0° and 28° of N. or S. lat.
"2ndly. It ought to be pointed directly towards the zenith of the place.
"3rdly. The projectile ought to be propelled with an initial velocity of
12,000 yards per second.
"4thly. It ought to be discharged at 10hrs. 46m. 40sec. of the 1st
December of the ensuing year.
"5thly. It will meet the moon four days after its discharge, precisely
at midnight on the 4th December, at the moment of its transit across the
zenith.
"The members of the Gun Club ought, therefore, without delay, to commence
the works necessary for such an experiment, and to be prepared to set
to work at the moment determined upon; for, if they should suffer this
4th December to go by, they will not find the moon again under the same
conditions of perigee and of zenith until eighteen years and eleven days
afterwards.
"The Staff of the Cambridge Observatory place themselves entirely at
their disposal in respect of all questions of theoretical astronomy; and
herewith add their congratulations to those of all the rest of America.
"For the Astronomical Staff,
"J. M. BELFAST,
"_Director of the Observatory of Cambridge._"