St. Tammany farmer. [volume] (Covington, La.) 1874-current, 1891-06-13
THE FIRST sExTANT.

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THE FIRST SExTANT.
History Of IN Important lsstroneat Ceea
In Navlgation-The Astrolabe.
The successful application Of the art
Of navigation depends almost as much on
the skill of the artists who construct tho
instruments employed as on that OF tho
observer who uses them. Many OF the
practical problems in daily use in the
navigation of our modern ships wero
known to our forefathers, but the inn
perfections of the instruments at their
command prevented making any great
use of them; indeed. it Is well knownto
those who have access to old works on
navigation that scarcely anything
really new has been brought into
practice during the last century, and We
are simply in most cases only awaiting
ourselves of our superior instruments
to bring into practical use what to our
ancestors was merely theory. The three
chief instruments employed in naviga-
ton are the compass, sextant and chro-
nOmeter. To understand the great inn
provement which the invention OF the
sextant made in the art of navigation
we must realize the kind of instrument
in use before Hadley's discovery in 1851.
The most careful Of the navigators OF
those days were provided with either
the back-staff, invented by Davis in
1890, or the astrolabc, an instrument OF
very great antiquity. The astrolsbe
was q circular metal disk, held in one
hand in q vertical position by a ring
from which was suspended q plumb line,
the upper left hand quadrant was grad
uated. and rod having . round hole at
each end worked on pivot as the CEN
ter Of the disk.
In taking an altitude of the sun the
instrument was held so that the plumb
line was over the center of the disk, and
the rod moved until the eye Mt the low
er end saw the sun through the hole at
the upper end, when the altitude could
be roughly read of, any thing less than
half degrees being estimated. Davis
back-staff (familiarly termed the ''hog's
yoke'O was . better instrument. In
using this the observer turned his back
to the sun and moved . small vane
traversing q circular wooden arc until
its shadow fell ON another vane kept
pointed at the horizon. With such
rough instruments as these il is not sur
prising that most sailors preferred their
dead reckoning to their astronomical
positions, and regarded any at
tempt at finding the longitude
rs hopeless, although theoretic-
ally the ..luna' had been known
for nearly two centuries. Sir Isaac
Newton appears to have been one OF
the first to see the advantage Of apply
ing the laws of reflection from glass
surfaces to the construction of an In
strument capable of measuring angles
between objects on any plane what
ever; and, doubtless from the manu-
scripts he left, had his life been pro-
longed, the great honor of the inven-
ton would have rested with him. As
it was, the same idea appears to have
been independently worked out prac-
tically by Hadley, gentleman OF
scientific tastes and considerable
means in England; and At the
same time by Godfrey, working
glazier in Philadelphia. How strange
that such valuable idea should have
occurred to two people at the same time.
Hadlcy's quadrant was very similar
to the instrument now in use, except
that the telescope was at first placed
parallel to radius Of the arc. and when
pointed to the horizon the limb Of the
quadrant was held in. an upright posi-.
ton. Hadley (who is also known to
sailors by his theory of the trade winds),
having perfected his instrument, read .
description of it before the Royal So
cicty, and the great value of the inven-
ton being ut once recognized BY Valley,
the astronomer royal, it was by him
brought to the notice Of the admiralty.
who ordered new trial of tito be made
at sea. Hadley, with his two brothers
and some members of the Royal society,
embarking for that purpose in the Chat
ham, q cutter of sixty tons.
For several days, in at times rough
weather, they cruised at the mouth OF
! Thames, taking many attitudes of the
| sun, which were compared with cal
culated attitudes depending upon the
times of observation, and the results. as
preserved in the proceedings of the
Royal society. show marvellous accord
It was q curious coincidence that the in
| vention of the quadrant, which for the
first time rendered possible the finding
of longitude by lunar, should have beer
almost Simultaneous with that of the
chronomcter, which soon rendered the
lunar method superfluous. But few
alterations have been made in the form
of the sextant since Hadley's time. The
| arc has been increased in length, 50 thai
| instead Of only measuring an angle
of ninety degrees as implied by
the name quadrant" modern ''scx
tants," - they are ter med. usually
| measure to about one hundred and thirty
degrees, and the arrangements for tax
| ing the back" observation, or supple
ment of the altitude, has been aban
doned. In other respects the modern
instruments resemble their ancient pro
totype, and It speaks volumes for the
saQacity Of an inventor who could pro
vide for the numerous purposes to which
his invention has been applied during
the last century and . half. The sex
tant, owing probably to its greater cost
did not come into general use for some
years, and in works on navigation fifty
wears later the astrolsbe is still de
scribed. A beautiful specimen of tha,
ancient instrument, used by Sir Franci.
Drake. Il preserved in the museum N
the royal naval college 1t Greenwich
Liverpool Journal of Commerce.