IV PREFACE. 



6th. An achromatic telescope of four feet in length, for 

 observing eclipses of Jupiter's satellites ; the magnifying 

 power I used for this purpose, was about 120. 



7th. A box chronometer, by Tobias and Levitt, London. 



8th. A pocket chronometer cased like a common watch, 

 by Brockbank, London. 



The rates of going of both these time-pieces will be found 

 among the Astronomical Calculations. 



9th. An instrument for observing the dip of the magnetic 

 needle, executed for the use of the Expedition, by Mr. I. 

 Lukens of Philadelphia, after an ingenious manner. It con- 

 sisted of a vertical circle of brass, graduated to half degrees, 

 in the centre of which a needle between nine and ten inches 

 in length, was suspended in the middle by two nicely polished 

 points resting in sockets. It was furnished with two spirit- 

 levels at right angles to each other, and leveling screws. In 

 order to observe the dip, the graduated vertical circle or 

 limb must be placed so that its plane shall coincide with that of 

 the magnetic meridian, which is done by means of a surveyor's 

 compass, or a common pocket compass will serve for this : 

 the instrument is then. levelled, and the dip read off on two 

 opposite points of the limb. By revolving the instrument 

 round on its vertical diameter, and again adjusting the levels, 

 two other readings may be obtained on opposite points of the 

 limb. Thus the mean of four observations may be taken for 

 the true dip, corrected by each other for any imperceptible 

 errors in the graduation of the limb, levelling of the instru- 

 ment, or eccentricity in the fixture of the needle in its sockets. 

 I conceive that the dip may be obtained within 5', although 

 the circle is only graduated to 30'. 



10th. An azimuth compass with a needle and card six 

 inches in diameter, graduated to half degrees, which might 

 be subdivided by the eye to 10'. The magnetic variation 

 was determined by observing with this instrument the mag- 

 netic azimuth and amplitudes of the sun. — Made by Richard 

 Patten, New York. 



11th. A common surveyor's compass, with a needle six 

 inches long. This instrument was used in tracing the course 

 of the Missouri, as laid down on the map of the country 

 taken by Major Long and Lieutenant Swift. — Made by the 

 same. 



12th. A fifty feet chain, with links each a foot long, con- 

 venient for travelling. — By the same. 



13th. A small theodolite, four and an half inches in dia- 



