Chapter XX (2)

Self-replenishing, 6in. long, 2-1/2in. broad, 3/4in. thick, weighs
1-1/4lb., cubic measure 11-1/4in.

The wooden one of olden date, 9-1/2in. long, 5-1/2in. broad, 5-1/2in.
thick, weighs 5-1/3lb., cubic measure 287-1/3in.

Improved folding roof, &c., all iron, 8in. long, 4-1/2in. broad,
2-1/2in. thick, weighs 6-3/4lb., cubic measure 90in.

The improvements herein specified are not only its reduced size and weight, but its mechanical arrangements, form, and moderate price.

It consists of two circular disc-like reservoirs, about 2-1/2in. in diameter, and 3/4in. in depth, made of iron, at the same casting: one contains the mercury, and the other is the trough, fitted with glass cover for observing.

The discs are connected at their circumference by a narrow neck, and in it is drilled a hole, through which the mercury passes from one reservoir to the other; and this communication is opened or shut off by a stop-cock, on the cone principle, such as is used for water or gas, so that the mercury can be passed from one disc to the other without removing the glass cover, or the risk of losing any mercury.

The mercurial disc, A, is fitted with a cylinder-stopper, D, acting on a spiral spring, by which air can be admitted or allowed to escape.

The trough disc, B, is fitted with two glasses, G and E, which are ground mathematically parallel: one of the glasses is fitted to a frame, and screws on the disc, and is used while passing the mercury in or out of the trough; after which operation it is removed and replaced by the other glass, the edge of which next the stop-cock should be supported by the blade of a pocket-knife and then lowered on the mercury at the opposite side, and by a gentle pressure, force out the intervening air, leaving the glass to float on the surface of the mercury. Without this care, some of the mercury might be pressed over the edge of the disc.

The glass then presents a clear reflecting surface, which is not only protected from the effects of the wind, &c., but also maintains so great a steadiness as to mark a decided improvement over the old triangular glass roof which is placed over the mercury, instead of, as in this case, being on it.

It may be used afloat under favourable circumstances (the observer and artificial horizon being placed on a pendulum table). Another great advantage to this improved artificial horizon is the facility with which altitudes can be observed at 2° elevation, and consequently its adaptation for the measurement of very low stars, as well as the peaks of mountain ranges.

To return the mercury to its reservoir, remove the glass G that floats on the mercury, by lifting it up with the point of a knife, and then screw on the other glass cover, E. It is now only necessary to hold the instrument vertically, the trough end being uppermost (Fig. 4), turn the stop-cock, and press gently downward on the cylindrical stopper, and the mercury will rapidly return to its reservoir.

The following diagrams (half the actual size of the instrument), show the various parts of the instrument, and the method of filling and emptying the reservoir.

Fig. 1 is the instrument complete. A, the mercurial reservoir; B, the observing trough; C, the stop-cock; D, the cylindrical stop.

Fig. 2 is the instrument with the parts of the observing trough removed, which are shown above it. E, rim with glass shade; F, rim without glass shade; G, the glass that floats on the surface of the mercury.

Fig. 3. Position of the instrument while filling the observing trough.

Fig. 4. Position of the instrument while returning the mercury into its reservoir.

In moderate weather the glass G will be quite sufficient protection against the wind, but in gusty weather screw on the rim F, but it must not touch the glass G.

The glass E will protect it from any weather, taking care to level the ground on which the horizon stands.

In filling the observing trough, be careful that the glass cover, E, is screwed on tight; by pressing on the cylindrical stop D (Fig. 1) the mercury flows quickly: the trough half filled, as shown in Fig. 3, is sufficient for ordinary observations; but for very low altitudes the trough must be three-quarters filled or more as found necessary to raise glass G (Fig. 1).

Before returning the mercury into the reservoir, unscrew the short tube near the stop-cock, tapping it smartly at the same time, to shake down the globules of mercury that may remain in the tube; there is a small hole in the screw, which must be brought in sight, then turn the stop-cock, and the mercury will run rapidly into the reservoir. When about to use a sextant and artificial horizon of the common form of construction, our first care is to select a tolerably level, and, if there be wind, a sheltered spot of ground, with a clear view to the north or south, or, if the stars admit of a north and south observation, to see that the view is clear both ways. On this we place our artificial horizon, sometimes on our sextant case, sometimes on a stand (wash leather), but seldom, if we can avoid it, on the bare ground, because then the mercury, if spilled, would be difficult to gather up. The horizon roof we keep near to cover the mercury, in case wind should arise, but we never use it unless in case of necessity; then, sitting either north or south of the horizon, according to the position of the celestial object, we look with the naked eye for its reflected image in the mercury, and so seat ourselves that we can conveniently keep it steadily in view. We set the sextant nearly to zero, and look up without the telescope to the sun or star, and then, gradually moving the index forward, we bring its image down to meet the reflection in the quicksilver; then, screwing on the inverting telescope, which is the simplest and best for observation, we move the index by hand, till the contact is nearly perfect; then fasten the index by the clamping screw, and with the tangent screw complete the contact; and so long as the object is rising, by gradually turning the tangent screw we keep the images together; when they separate more slowly, and at length remain in contact for nearly half a minute, we know that the meridian altitude has been observed; we wait another minute to see them separate in the opposite direction as the body begins to descend, and then read off the observed altitude. Our illustration will sufficiently explain that the sextant is held in the left hand and the tangent screw worked with the thumb and forefinger of the right. Fig. 2 is the method recommended by Captain George: the arc is steadied by the forefinger, and the tangent screw turned by the middle finger and the thumb; a police or bull's-eye lantern is good to read off by, and the light, of whatever nature, should be placed behind the observer, so that it may not interfere with his work, and yet may be ready for him to use when he wishes to read his altitude.

PROJECTION OF ROUTES.

The following directions for the projections of routes by Captain George, R.N., are so thoroughly plain and practical that it will be well for the travelling observer to have recourse to them:--For out-door or field work the easiest method is by the plane projection, the data thus obtained being transferred to a Mercator's projection at the first halt or stopping station. In the plane projection one equal length is assigned to all the degrees of latitude and longitude. It was first adopted on the erroneous supposition that the earth's surface is a plane; it is still the best for the traveller to use in his early attempts to project his journey while the objects are still in sight. This projection is available as far as 20° on either side of the Equator; beyond the parallel of 20° and as far as 60° Mercator's projection is preferable. Between 60° and the pole the distortion of both the plane and Mercator's projection is so apparent, that a polar or circular projection must be adopted. Sheets of paper, ruled into squares by strong lines, and subdivided by finer ones, afford great assistance in map work. For out-door work the scale of 1in. to one mile is amply large enough to register every particular of one day's journey on a sheet of 12in. square. The in-door, or table plan, may be reduced ten miles to the inch, and plans for transmission home maybe again reduced to 1in. to 1° when larger plans cannot be sent. The chief point aimed at in the following directions is to draw more attention than has hitherto been given to the true bearing of objects, for the following reasons: First. Any object whose true bearing is east or west must be in the same latitude as the place of the observer. Secondly. Any object whose true bearing is north or south must be in the same longitude as the place of the observer.

While travelling in a northerly or southerly direction, from a station whose latitude is known, and carefully noting the distance and direction travelled, it is only necessary to watch when objects come to the true east or west, and their latitude is obtained. When travelling in an easterly or westerly direction from a fixed station, noting distance and direction, it is only necessary to watch when objects come to the true north or south, and their difference of longitude can be obtained by using Table B, from the station left. Thus, suppose a traveller passes from A, whose latitude is known, towards some distant hill (B), his route making an angle of 25° with the meridian. He sets his sextant to 65° (65° + 25° = 90°), or to 115° (180° - 65°); then as the objects 1, 2, 3, and 4 successively come into contact with B or A, as the case may be, he ascertains with precision the moment when they are truly east or west of him, and so, knowing the distance he has travelled from A, he can readily calculate or project their latitude.

When the traveller, as will frequently be the case, has to deviate from the line of route, his position can be determined by compass, or true bearing of any object, and an angle of a second object; or he may have recourse to transit observations; that is to say, wherever two fixed objects come in line, an angle to a third object will determine the position with great accuracy.

Observe that in travelling along X Y Z the hills A B C can be mapped for at X, or thereabouts; the bearing of B from C can be determined at Y; that of A from B; and at Z that of A from C, and so on, for any number of hills. And it is very important to recollect that it is not necessary to catch these lines of sight precisely, for by taking bearings twice, and the intermediate course approximately, there are sufficient data for protracting out upon paper the required bearing.

Thus, as soon as the peak of a distant hill is about to be occulted by the shoulder of a nearer one, a bearing should be taken, and again another as soon as it has reappeared on the other side, and the intermediate course noted.

The advantage of this method of filling up a field sketch will become more apparent as experience is gained. A third and accurate method of fixing the position is in general use among marine surveyors, but has hitherto been but little resorted to by land travellers, viz., by the angles subtended between three known objects. The instrument called the station-pointer is generally used for this purpose, but the position may also be found with a pair of compasses and a protractor, or more simply as follows by means of a protractor and a sheet of tracing paper. Draw a line through the centre of the paper, place the protractor on it, near to the bottom of the sheet, lay off the right hand angle to the right, and the left hand angle to the left of the centre line; rule pencil lines, radiating from the point over which the centre of the protractor had been placed, to the points that had been laid off, then place the paper on the plan or map, and move it about until the three lines coincide with the objects taken, prick through the points that lay beneath the centre of the protractor, and the observer's position is transferred to the plan. When possible the centre object should be the nearest.

TO CONSTRUCT A MAP ON MERCATOR'S PROJECTION.

On a sheet of cartridge paper, 38in. by 20in., it is proposed to construct a map on Mercator's projection, on a scale of ten miles to an inch equatorial, _i.e._ 6in. to a degree of longitude.

Lat. 31° to 33° N.
Long. 34° to 36° E.

Draw a base line, find its centre, and erect a perpendicular to the top of the paper; the extremes of longitude 34° and 36°, added together and divided by 2°, give 35°, the central meridian, and which is represented by the perpendicular. On each side of it lay off 6in., and erect perpendiculars for the meridians 34° and 36°; divide the base line into ten mile divisions, and the part from 35° 50´ to 36° into miles for the latitude scale. From Table A take the following quantities:--

° ° ° ´ ° °
Lat. 31 to 32 = 1 10·4 = the distance between parallels 31 and 32
Lat. 32 to 33 = 1 11·1 " " 32 and 33
-------
2 21·5 " " 31 to 33

Having thus obtained the distance between the required parallels, divide the map into squares of ten miles each way, and the map is ready for the projection of the route.

A.--TABLE TO CONSTRUCT MAPS ON MERCATOR'S PROJECTION.

--+------+------+------+------+------+--------+-------+-------+-------+-------
| 0° | 1° | 2° | 3° | 4° | 5° | 6° | 7° | 8° | 9°
--+------+------+------+------+------+--------+-------+-------+-------+-------
|° ´ |° ´ |° ´ |° ´ |° ´ | ° ´ | ° ´ | ° ´ | ° ´ | ° ´
0| |1 00 |1 00·1|1 00·1|1 00·1| 1 00·2 | 1 00·3| 1 00·4| 1 00·5| 1 00·6
10|1 00·9|1 01 |1 01·2|1 01·5|1 00·7| 1 02 | 1 02·2| 1 02·6| 1 02·9| 1 03·3
20|1 03·6|1 04·1|1 04·5|1 04·9|1 05·5| 1 05·9 | 1 06·5| 1 07 | 1 07·7| 1 08·2
30|1 09 |1 09·6|1 10·4|1 11·1|1 12 | 1 12·8 | 1 13·7| 1 14·6| 1 15·7| 1 16·7
40|1 17·6|1 19 |1 20·1|1 21·4|1 22·7| 1 24·2 | 1 25·6| 1 27·1| 1 28·8| 1 30·6
50|1 32·4|1 34·3|1 36·4|1 38·6|1 40·8| 1 43·4 | 1 45·9| 1 49 | 1 51·4| 1 54·8
60|1 58·3|2 01·8|2 05·8|2 09·9|2 14·5| 2 19·14| 2 24·7| 2 30·5| 2 36·8| 2 43·8
70|2 51·3|2 59·8|3 09·1|3 19·6|3 31·3| 3 44·6 | 3 59·8| 4 17·1| 4 37·4| 5 01·1
80|5 29·5|6 03 |6 46·4|7 40·3|8 51·1|10 27·7 |12 47·9|16 29·6|23 14·3|39 42·2
--+------+------+------+------+------+--------+-------+-------+-------+-------

_Use of the table._--Find the required parallel; the tens at the side, and the units at the top. At their intersection will be found in degrees and minutes the distance of the required parallel from the next less degree, to be measured from the scale of longitude on the map in progress.

Given the parallel of 30°, required that of 31°. 30 at the side and 1
at the top intersects at 1° 09·6´, the required distance of the two
parallels.

Given the parallel of 31°, required that of 33°:

° ° ´
32 = 1 10·4
33 = 1 11·1
------
2 21·5 the distance between the 31° and 33° parallels.

B.--GIVEN THE DEPARTURE TO FIND DIFFERENCE OF LONGITUDE.

--+------+------+------+------+------+-------+-------+-------+-------+-------
| 0° | 1° | 2° | 3° | 4° | 5° | 6° | 7° | 8° | 9°
--+------+------+------+------+------+-------+-------+-------+-------+-------
0| |1·0001|1·0006|1·0013|1·0026| 1·0038| 1·0055| 1·0075| 1·0098| 1·0125
10|1·0154|1·0187|1·0224|1·0261|1·0306| 1·0353| 1·0403| 1·0457| 1·0514| 1·0578
20|1·0642|1·0711|1·0785|1·0864|1·0946| 1·1034| 1·1126| 1·1224| 1·1326| 1·1434
30|1·1547|1·1666|1·1792|1·1924|1·2062| 1·2208| 1·2361| 1·2521| 1·2690| 1·2868
40|1·3054|1·3250|1·3456|1·3673|1·3902| 1·4142| 1·4395| 1·4663| 1·4945| 1·5242
50|1·5557|1·5890|1·6242|1·6616|1·7013| 1·7435| 1·7883| 1·8361| 1·8871| 1·9416
60|2·0000|2·0626|2·1301|2·2027|2·2812| 2·3662| 2·4586| 2·5593| 2·6695| 2·7904
70|2·9238|3·0716|3·2361|3·4204|3·6280| 3·8637| 4·1337| 4·4454| 4·8097| 5·2406
80|5·7587|6·3925|7·1856|8·2057|9·5664|11·475 |14·334 |19·108 |28·653 |57·307
--+------+------+------+------+------+-------+-------+-------+-------+-------

_Use of the table._--Find the required parallel, the tens at the side and the units at the top, at their intersection will be found a quantity, which, multiplied by the departure, gives the difference of longitude.

The departure from the meridian on the parallel of 34° was 25 miles,
required the difference of longitude:

25´ × 1·20 = 30·00´ the difference of longitude.

In the parallel of 60° the departure was 30 miles:

30´ × 2 = 60 miles, or 1 degree.

In the parallel of 35° N. the route was N. 40° W. 37 miles distance.

By traverse table, 40° course, dist. 37° = dep. 23·8´ × 1·22 = 29·03
miles difference of longitude.

The following example will serve to show how the traveller's record of progress may be conveniently kept. It was framed by S. W. Norie expressly for the use of navigators; but explorers and travellers will find it a simple and useful form for recording the day's work:

------------------+-----------+-------------------------+-------------
Corrected | | Difference of Latitude. | Departure.
Courses. | Distance. +------------+------------+------+------
| | N. | S. | E. | W.
------------------+-----------+------------+------------+------+------
N.E. | 36 | 25·5 | | 25·5 |
N. by W. | 14 | 13·7 | | | 2·7
N.E. by E. 1/2 E. | 58 | 27·3 | | 51·2 |
N. by E. | 42 | 41·2 | | 8·2 |
E.N.E. | 29 | 11·1 | | 26·8 |
| +------------+ +------+------
| Difference| 118·8 | |111·7 | 2·7
| of Lat. | | | 2·7 |
| | | | -- |
| | | Dep. |109·0 |
------------------+-----------+------------+------------+------+------

The difference of latitude 118·8 and departure 109·0 give the course
N. 42° 32´ E., and distance 161·2.

° ´ ° ´
Latitude left 52 36 N. Merc. prjctns. 3724 Longitude left 21 45 W.
Diff. lat. 1 59 N. Diff. lon. 184 or 3 4 E.
-- -- -----
Latitude in 54 35 N. Merc. prjctns. 3925 Longitude in 18 41 W.
------ ----
Sum of lats. 2)107 11 Merc. diff. lat. 201

Mid lat. 53 35

TO MEASURE THE NUMBER OF CUBIC FEET OF WATER CONVEYED BY A RIVER IN EACH SECOND.

In traversing regions watered by rivers and running streams, it not unfrequently becomes important to ascertain the speed at which they flow in their downward course towards the sea, and the following directions given by Captain George, R.N., are so perfectly clear and practical that both time and trouble will be saved by the traveller who follows them out in conducting his investigations. The data required are--the area of the river, section, and the average velocity of the whole current. All that a traveller is likely to obtain without special equipment is the area of the river, section, and the average velocity of the "surface" of the current which differs from that of its entire body, owing to fractional retardation at the bottom.

To make the necessary measurements, choose a piece where the river runs steadily in a straight and deep channel and where a boat can be had. Prepare half a dozen floats of dry bushes, with paper flags, and be assured they will act. Post an assistant on the river bank at a measured distance (of about 100 yards), down stream, in face of a well-marked object; row across stream, in a straight line, keeping two objects on a line in order to maintain your course. Sound at regular intervals from shore to shore, fixing your position on each occasion by a sextant angle between your starting place and your assistant's station, and throw the floats overboard, signalling to your assistant when you do so, that he may note the interval that elapses before they severally arrive opposite him. Take an angle from the opposite shore to give the breadth of the river. To make the calculation approximately, protract the section of the river on a paper, ruled to scale in square feet, and count the number of squares in the area of the section. Multiply this by the number of feet between you and the assistant, and divide by the number of seconds that the floats occupied on an average in reaching him.

Important rivers should always be measured above and below their confluence, for it settles the question of their relative sizes, and throws great light on the rainfall over their respective basins. The sectional area at the time of the highest water, as shown by marks on the banks and the slope of the bed, ought also to be ascertained.

ON OBTAINING GEOGRAPHICAL INFORMATION FROM NATIVES OR FRONTIER
COLONISTS.

Many highly-accomplished travellers fail to obtain much reliable information beyond the actual limit of their own observation, because they do not sufficiently allow for the great difference in the manner of expressing a geographical idea between an educated European observer and an untutored savage; and yet it would not be too much to say that the latter has often enough a thoroughly practical idea of the district he actually knows.

The man who wants information must not talk latitude and longitude to a native or to an uneducated European, nor must he expect them to shape their answers to the form in which he expects to receive them; for if he does he may be told that "rivers run from the sea to the mountains," and other absurdities, which are related as proofs of native stupidity, when they are in reality no more than discrepancies between the form of question and that of the answer. At the same time he must estimate the mental calibre of his informant, and avoid wearying him too much; for sometimes the native mind, over burdened with a succession of ideas, becomes confused, and not unfrequently suspicious, and in this last case actual falsehoods will be told, in order to gain time to find out the intention of the questioner, before the truth is revealed.

In all dealings with natives the European must remember that they have no idea of the value we place upon time; there is no use in saying, "Let us come to the point at once." It is far better, indeed it is absolutely necessary, to delay judiciously, as there is always an implied contest between visitors; and the man who is in a hurry to speak loses dignity. Do not disturb your informant's train of thought, but try to accommodate your own to it. Let him tell as minutely and tediously as he pleases how he has travelled; how long he walked with the rising sun on his right or left; how much he turned either way; where he halted for rest or refreshment; whether he crossed rivers on foot or in canoes; induce him, if possible, to trace a map upon the ground, in doing which he will most probably begin by making the direction of all his lines coincide with the actual bearing of the country; for natives, though they may be brought to comprehend a map when its north point coincides with the real north, cannot believe that it is also right when it is placed in any other position. We have frequently tested Hottentots with regard to the direction of places a thousand miles distant, and have found them point as correctly as we could take the bearing with a pocket compass.

Sometimes it will be found that the same individual will give a river a dozen different names; and this is often because, in speaking of the different parts of it, he gives to each the name of the chief who has his village there, and who "drinks water" at the place which is called after him; therefore, endeavour to ascertain whether the river has a real name, and do not apply to it the first and, perhaps, the most inappropriate name that is given.

Europeans who have settled in the colonies, and have become traders or hunters, frequently push very far into the interior, and such men have generally very clear ideas of direction and locality, but are often very modest and diffident when asked to furnish information to be laid down on paper. In 1849, while staying at the Vaal River, we persuaded our friend Macabe, with some difficulty, to give us the length in "hours" and the direction of the various stages of his journey on the river Limpopo. These, with the rivers, mountains, villages, and other features of the country, we laid down, on a scale of one inch to a mile, on several sheets of cartridge paper, and tested the correctness of our work by laying it on the floor, with its north coinciding with the real north, and requesting our Dutch visitors to stand as if they were about commencing the same journey, and indicate how much they turned to the right or left as they proceeded.

The following facts relating to time should be impressed on the memory of every traveller:--The earth is divided in its circumference into 360°; the day is divided into twenty-four hours. It therefore follows that 15° of longitude will represent one hour of time; consequently, as you travel towards the east, when you have journeyed over 15°, you will have gained one hour on the sun, which will rise just one hour earlier than it did at the starting point.

If natives accompany you on a journey, ask them to point in the direction of places at different times, and particularly to tell you when you are exactly abreast of them, as well as before, and after you have passed, and thus, by a kind of rough triangulation, you will gain their approximate position.

If cattle stray to great distances, ask the men who go after them the reason of their having taken any particular direction, and you will probably gain some information respecting the form of valleys or mountains, and perhaps of watering places.

In North Australia we were led by the tracks of horses, which had been lost about a fortnight, to a considerable stream.