(3.) To laying out a direct line between any two objects, thus acting as a substitute for a Raper’s instrument.
(4.) To laying out curves for railways, harbour works, &c., &c.
(5.) It can be used as an optical square.
(6.) It can be used as a dip-sector.
(7.) It can be used on shipboard to measure the supplement of the meridian altitude, in cases where the land intervenes between the observer and the direct meridional horizon.
(8.) It can be used on shore with the artificial horizon in obtaining altitudes of objects near the zenith.
(9.) It is also available as two distinct sextants, one of which can be used in case of the other being damaged, or one can be used by an assistant, and the other retained by the observer.
A pocket compass is now made in which the northern half is black with white points, and the southern white with black points; the advantage of which by night or twilight is obvious. We prefer that the card should travel with the magnet, as all the points then come naturally into position, and the excessive liveliness of the needle which renders a rapid observation so difficult is obviated. We have carried a pocket compass with a swivel ring, so that it could be worn on the left thumb, while we held the note book and kept the right hand free for writing or guiding the horse. It is easier to make pencil notes on horseback than in a waggon. For the proper registration of the time and distance travelled, a good well-going hunting watch is necessary; and if it has a black dial and white figures so much the better. This will serve sufficiently well for taking time in the observation of lunar distances. Unless under very exceptional circumstances it would be useless for an explorer to trouble himself with a chronometer. A good binocular field glass for day and night will be found useful.
PORTABLE OBSERVATORY.