54 DETERMINATION OF ELEMENTS OF MAGNETISM AT CAMPS. 
its use in obtaining the magnetic inclination of the needle, and relative intensity of magnetism 
at different points of the earth’s surface. The transit was made by Troughton & Simms, of 
London, after a plan furnished by Col. Graham, of the Topographical Engineers. Upon three 
foot-screws rests a circular base, to which are attached, by movable screws, the vertical up- 
rights forming the Y’s. The telescope has a focal length of twenty-two inches. The whole is 
light and portable. We have had a pine stand made for it, the parts firmly fastened together 
with wooden pins. Iron was excluded, in order that it might serve also for the magnetic in- 
strument. Lieut. Ives was directed to commence a series of observations at this place, to serve 
as a basis for comparison with succeeding stations. In none of our text-books can be found 
detailed instructions upon the mode of adjusting and using these instruments with the rapidity 
required for field service. As, hereafter, only slight notice will be taken of this portion of our 
operations, it may not be deemed out of place to insert the method which experience, upon sim- 
ilar surveys, had suggested as proper to adopt. It will serve also to show what value should 
be accorded to the results obtained from the observations. 
MODE OF DETERMINING THE ASTRONOMICAL POSITIONS AND THE ELEMENTS OF MAGNETISM AT TEMPO- 
RARY CAMPS, 
Upon arriving at camp, usually from 3 to 5 p. m., a firm stool, about two and a half feet 
high, will be placed on solid ground, from whence a clear view of the heavens, and particularly 
of the meridian, can be obtained. A trench from one and a half to two feet deep will be dug 
surrounding the stand, about eighteen inches from the point beneath the centre, leaving an iso- 
lated column of earth, free from the vibratory motion communicated by the ordinary move- 
ments of the men and animals about camp. There should be a platform for the observer north 
and south of the stand, resting entirely outside the trench. It must be recollected that the ` 
value of the observations greatly depends upon the isolation of the instrument. Hence a flat 
rock should never be selected as a foundation, in case the observer is obliged to stand upon the 
same himself. Cooking-fires should be at least 300 feet distant, and to the leeward, that the - 
smoke may not vitiate the results. 
The stand being prepared, the ** Fox” magnetic dip and intensity instrument will be placed 
upon it and adjusted; the recorder will take the note-book; the instrument-attendant his ivory 
disc, and the observer make the usual observations upon the needles and azimuth circle for the 
magnetic meridian, inclination, and intensity. "This operation, and a record of the usual bar- 
ometric and meteorological observations, will be completed at sunset. With the estimated lati- 
tude of the place, the telescope of the dip-circle will be set to the altitude of Polaris, which will 
be observed at the intersection of the two wires as soon as it appears, and the time of observa- 
tion and readings of altitude and azimuth circles recorded. The telescope will be now depressed 
to the same angle below the horizon, and the star observed as reflected from an artificial hori- 
zon of mercury. The face of the instrument being set in the opposite direction, similar obser- 
vations on Polaris will be recorded; first by reflection, and then direct. We have now, provided 
the error of chronometer be known, data for determining the reading of the azimuth circle when 
the telescope is in the true meridian; which, compared with observations previously recorded for 
the magnetic meridian, gives, approximately, the magnetic declination. The altitudes read 
will give the latitude of the place to the nearest minute. 
The magnetic instrument, the adjustments of which have remained undisturbed during the 
preceding operations, now gives place to the astronomical transit. Polaris being visible, the 
first approximation to the meridian will be to direct the telescope towards that star. By the 
striding level the stand will be approximately levelled, and the telescope again directed towards 
Polaris, if supposed near its upper or lower culmination. If the error of the chronometer be 
entirely unknown, adjust the axis by the foot-screws, until the reversals of the striding level 
prove that the telescope moves in a vertical plane; then elevate, and observe the chronometer 
time of passage over the middle wire of the first known star near the zenith. The difference 
