The Journals of Lewis & Clark: Lewis, July 22, 1804

Updated May 14, 2020 | Infoplease Staff

Lewis, July 22, 1804

July 22nd

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.

1st- a 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.

2ed- A 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.

3rd- An 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.

4th- An 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.

5th- An 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

6th- A 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.

7th- A 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.