TRIANGULAR COMPASSES.
SLIDING BEAM COMPASSES.
Portable observatory.
Measurement of distances.
We give a sketch of a very convenient arrangement made for us by Mr. Casella, in which the roof of the artificial horizon, slung with its point downward in a leather case, with pieces of tin let in to protect the glass from injury, was filled up with a block of light cedar, with hollows cut in it for the reception of the pocket sextant, iron-stoppered bottle of mercury and funnel cap, prismatic compass, note book with tables of declination cut from Hannay and Dietrichsen’s Almanac pasted in it, pencil, skin of chamois leather, and over all the horizon trough. Lieutenant Skead, R.N., who accompanied the expedition to the Zambesi, frequently used it, and called it “a portable observatory.” If the traveller intends to be long absent, he should supply himself with the Nautical Almanac for three years in advance, as well as with Norie’s or Raper’s Epitome, or Kerigan’s Navigation. In addition to the instruments we have described, triangular compasses are very useful for taking the exact relative position of three points, and sliding beam compasses for long distances. Proportional compasses are also very useful in plotting the result of observations; we were accustomed to pin down half a dozen sheets over each other on the drawing board, and with a fine needle point prick the course through the whole of them; then by underlaying them with semi-carbonic paper, and writing the names on the upper sheet with a HHH. pencil, three or even more copies at a time might be obtained, the number, of course, depending much on the thinness of the paper we worked on.
(See accompanying illustration.) Measurement of distances. Bear in mind that what an explorer wants is the means of approximately laying down his course and distance travelled, and his latitude precisely; the sextant and artificial horizon will do the last within a mile. The compass will give the course very nearly if he walks or rides; no instrument can be perfectly depended on. A pedometer will do for short distances, but when he becomes weary it counts his feeble steps just as it did his vigorous strides at the beginning, and thus shows more than the truth. If wheel carriages can be used, take a trochiameter, and contrive if possible to have the wheel on which it is fixed exactly five yards in circumference; it saves no end of trouble if there are no odd half inches to calculate. For mapping, do not take a case of instruments unless they are really good; have rather a few good ones wrapped in a chamois skin, a small ivory rule on which the eighths of an inch serve for miles, a pair of good dividing compasses, a good circular protractor transparent marked, from 0 to 360, a small parallel rule, HHH. pencils, a cake of blue for rivers, and carmine for roads, with a couple of sable pencils and a fine incorrodible metallic pen, will enable you to make a very complete and reliable map.