The same officer describes an ingenious substitute for an ordinary water level, which can always be replaced by a little ruler, A B, suspended by strings, C A, C B, having a little weight under it to prevent the wind from shaking it. When held by the string the line A B will give a horizontal direction. To make use of it for levelling along A B, start from A, hold the ruler up to the eye, and, aiming along its edge, notice to what point (B) of the ground the visual ray corresponds. Repair there, we shall have ascended a distance—the height of the eye above the ground. Start afresh from B, and in this manner the number of stations made between A and B, multiplied by the height of the eye above ground, will give the difference of level required.
The same author gives the following useful hints on the estimation of distance: “Pacing is generally resorted to while filling in the details of a survey. The trotting of a horse might also be made available. Distances can also be measured by time when we have previously ascertained over how many yards we walk or ride in a given time. This is not a rare occurrence in the field. When distances are measured by pacing or riding a correction is necessary, owing to the lengthening caused by acclivities and the turnings of roads. On slightly uneven ground we subtract 1-7th of the distance paced, and 1-5th when the inductions are more important.”
When the atmosphere is even sound travels at the rate of 1118ft. per second; therefore a musket fired may serve to measure a distance. A watch gives the number of seconds elapsed between the instant the light is seen and that when the report is heard; that number, multiplied by 1118ft., gives very approximately the distance. If no watch is to be had the time is obtained by counting the pulsations of an artery. A sound pulse averages from 75 to 80 in a minute. Distances may even be guessed by observing that in clear weather the windows of a house can be counted at 4000yds. Horses and men appear as dots at 2200yds. A horse is clearly seen at 1200yds. The movements of men are perceived at 800yds.; and the head is distinctly visible at 400yds.
For the measurement of time, as we have said before, a thoroughly well-made English lever watch is preferable to a chronometer, as it will withstand the rough jolts and vibrations caused by being carried on horseback, or in a waggon, far better. Sun-dials, properly so called, are rarely of much use to the traveller; still the pocket compass may be made to do duty as an indicator of time. There is a small and very portable little instrument to be obtained of most opticians, in the formation of which a silk cord is so attached to the border of the compass case that on adjusting the compass, so that the needle shall point to a black stud inserted for the purpose, a shadow is cast by the silk on the figure indicating the hour of the day. A small equation table is attached to the inside of the cover.
When no watch is at hand seconds can be indicated closely enough for practical purposes by suspending a small bag of shot or bullets from the end of a piece of fine fishing line or copper wire. Attach the upper end to a cross bar, laid in the crutches of two forked sticks, let the bag hang, and regulate the length of the string until you find that it describes the proper arc in swinging; your own pulse, the number of which has been already given, or that of a horse, which may be roughly set down as thirty-six beats per minute, will be a sufficient guide. If great accuracy is sought, recourse may be had to repeated astronomical observations.
An excellent makeshift hour-glass can be made from two empty soda-water bottles, and a little fine dry sand. A wooden plug of 3in. long should be so cut as to fit the necks of the bottles tightly. Through the centre of the plug, from end to end, burn with a red-hot wire a fine even hole, then with your pocket-knife make a funnel-shaped or flared-out mouth to each hole, cutting away until the extreme edge of the plug is reached. See that your sand is free from small stones or lumps, pour it into one of the bottles, insert the plug half-way, and test the quantity by letting it run out at the hole. When you have the proper charge to run for fifteen or thirty minutes, place the bottles mouth to mouth in such a way that one-half the plug shall be in the neck of each bottle. A bit of raw hide sewn round the union of the bottle mouths makes all secure. The joined bottles can then be mounted in a wooden frame for use. Two bits of flat square board, with holes in the centre for the bottoms of the bottles to come partly through, pinned at the corners by four bars of wood, is as good a form of frame as any.
The Malays make use of a very convenient and simple form of time indicator or water clock, which is made as follows:—A large-sized cocoa-nut shell is obtained; this is first scraped perfectly smooth, and then at the bottom a very minute hole is bored. The nutshell is then set floating in a pail of sea water. As the shell fills it gradually settles deeper and deeper, and at last sinks to the bottom with a gurgle and a thud. The rapidity of filling, and consequent duration of time, is regulated by increasing or diminishing the size of the orifice. Thus a man may be set to keep a two-shell watch or a four-shell watch, and so on. The instant the shell sinks to the bottom of the pail it attracts attention by the disturbance made. It is then immediately taken up, the water is poured out, and it is set afloat again. Excellent time can be kept by this primitive arrangement.
For ascertaining the altitude of high lands, ranges of hills or mountains, a thoroughly good “compensated” aneroid barometer should be taken. This will not only be valuable for measurements, but will be of considerable service in the observation of weather signs. We have one now in use made expressly for us by Mr. Cary, of 181, Strand. It is protected by a smooth wooden cover or case, enveloped in tightly-stretched leather. A sling is fitted to it by a swivel loop, which admits of its being carried over the shoulder or in the jacket pocket. The Table of Altitudes, given on the next page, will prove a useful guide when conducting observations with it.