Lewis: July 22, 1804
July 22nd 1804. A summary discription of the apparatus employed in the
following observations; containing also some remarks on the manner in
which they have been employed, and the method observed in recording the
observations made with them.
1sta brass Sextant of 10 Inches radius, graduated to 15 which by
the assistance of the nonius was devisible to 15″; and half of this sum by
means of the micrometer could readily be distinguished, therefore-7.5″ of
an angle was perceptible with this instrument; she was also furnished with
three eye-pieces, consisting of a hollow tube and two telescopes one of
which last reversed the images of observed objects. finding on experiment
that the reversing telescope when employed as the eye-piece gave me a more
full and perfect image than either of the others, I have most generally
imployed it in all the observations made with this instrument; when thus
prepared I found from a series of observations that the quantity of her
index error was 8′ 45″-; this sum is therefore considered as the standing
error of the instrument unless otherwise expressly mentioned. the
altitudes of all objects, observed as well with this instrument as with
the Octant were by means of a reflecting surface; and those stated to have
been taken with the sextant are the degrees, minutes, &c shewn by the
graduated limb of the instrument at the time of observation and are of
course the double altitudes of the objects observed.
2edA common Octant of 14 Inches radius, graduated to 20′, which by
means of the nonius was devisbile to 1′, half of this sum, or 30″ was
perceptible by means of a micrometer. this instrument was prepared for
both the fore and back observation; her error in the fore observation is
2°+, & and in the back observtion 2° r 1′ 40.3″ + at the time of our
departure from the River Dubois untill the present moment, the sun’s
altitude at noon has been too great to be reached with my sextant, for
this purpose I have therefore employed the Octant by the back observation.
the degrees ‘ & “, recorded for the sun’s altitude by the back
observation express only the angle given by the graduated limb of the
instrument at the time of observation, and are the complyment of the
double Altitude of the sun’s observed limb; if therefore the angle
recorded be taken from 180° the remainder will be the double altitude of
the observed object, or that which would be given by the fore observation
with a reflecting surface.
3rdAn Artificial Horizon on the construction recommended and
practiced by Mr. Andrw. Ellicott of Lancaster, Pensyla., in which water is
used as the reflecting surface; believing this artificial Horizon liable
to less error than any other in my possession, I have uniformly used it
when the object observed was sufficiently bright to reflect a distinct
immage; but as much light is lost by reflection from water I found it
inconvenient in most cases to take the altitude of the moon with this
horizon, and that of a star impracticable with any degree of accuracy.
4thAn Artificial Horizon constructed in the manner recommended by
Mr. Patterson of Philadelphia; glass is here used as the reflecting
surface. this horizon consists of a glass plane with a single reflecting
surface, cemented to the flat side of the larger segment of a wooden ball;
adjusted by means of a sperit-level and a triangular stand with a
triangular mortice cut through it’s center sufficiently large to admit of
the wooden ball partially; the stand rests on three screws inserted near
it’s angles, which serve as feet for it to rest on while they assist also
in the adjustment. this horizon I have employed in taking the altitude of
the sun when his image he has been reather too dull for a perfect
reflection from water; I have used it generally in taking the altitude of
the moon, and in some cases of the stars also; it gives the moon’s image
very perfectly, and when carefully adjusted I consider it as liable to but
little error.
5thAn Artificial Horizon formed of the index specula of a Sextant
cemented to a flat board; adjusted by means of a sperit level and the
triangular stand before discribed. as this glass reflects from both
surfaces it gives the images of all objects much more bright than either
of the other horizons; I have therefore most generally employed it in
observing the altitudes of stars
6thA Chronometer; her ballance-wheel and escapement were on the
most improved construction. she rested on her back, in a small case
prepared for her, suspended by an universal joint. she was carefully wound
up every day at twelve oclock. Her rate of going as asscertained by a
series of observations made by myself for that purpose was found to be 15
Seconds and a 5 tenths of a second too slow in twenty four howers on Mean
Solar time. This is nearly the same result as that found by Mr. Andrew
Ellicott who was so obliging as to examine her rate of going for the space
of fourteen days, in the summer 1803. her rate of going as ascertained by
that gentleman was 15.6 s too slow M. T. in 24 h. and that she went from 3
to 4 s. slower the last 12 h, than she did the first 12 h. after being
wound up.
at 12 OCk. on the 14th day of may 1804 (being the day on which the
detachment left the mouth of the River Dubois) the Chronometer was too
fast M. T. 6 m. 32 s. & 2/10.This time-piece was regulated on
meantime, and the time entered in the following observations is that shewn
by her at the place of observation. the day is recconed on Civil time, (i
e) commencing at midnight.
7thA Circumferentor, circle 6 Inches diameter, on the common
construction; by means of this instrument adjusted with the sperit level,
I have taken the magnetic azimuth of the sun and pole Star. It has also
been employed in taking the traverse of the river:from the courses
thus obtained, together with the distances estimated from point to point,
the chart of the Missouri has been formed which now accompanys these
observations. the several points of observation are marked with a cross of
red ink, and numbered in such manner as to correspond with the celestial
observations made at those points respectively.
