To all, however, who have the means, we would say do not fail to buy a trocheameter: it is a small, compact instrument, fitted in a copper case, capable of being strapped on any convenient part of the wheel; and one of fair quality need not cost above 2l. 10s. or 3l. The instrument is composed of two revolving toothed wheels, the upper wheel having 101 and the lower 100 teeth, suspended from and turned by an endless screw; there are two indices, that on the upper wheel pointing out every single revolution, and that on the lower every hundred. The whole circuit of the instrument is 10,100 revolutions, and the following is an example of its power:
“One complete circuit of 10,100 revolutions, with a carriage-wheel of 12ft. circumference, would indicate 23 miles, minus 80yds. Thus, 55 revolutions give 220yds., or 1 furlong; 110 give 440yds., or a quarter of a mile; 440 give 1760yds., or 1 mile; 7040 give 16 miles; 10,100 equal to 23 miles, minus 80yds.
“To set the instrument unscrew the milled nut from off the steel endless screw, and move the wheels round until both the indices coincide; screw the nut firmly in its place, shut up the instrument, and strap it securely to the off-wheel in the centre of the nave.”
In Africa we cannot literally follow out these instructions, for the nave is not brass capped, as with carriage-wheels at home, but the end of the axle comes through, and the wheel is secured to it by a washer and a linch pin; therefore, we strap the trocheameter between the spokes as near to the nave as possible, and in our journey to the Zambesi Fall we secured a pint pannikin permanently between the spokes as a protection to the trocheameter, which just fitted nicely into it during this journey. We measured a distance of between 2000 and 3000 miles, and do not remember that this instrument failed, except once from being choked with fine dry sand, and once again from equally fine sand and water.
We subjoin a table, by which it will be seen that our waggon-wheel was 5yds. 2½in. in circumference; this fraction gave some little trouble in the preliminary computation, and it looked very absurd to calculate the stages to half an inch, but if we had thrown it out a considerable error would have accumulated, and when the table was once formed the trouble was at an end.
Trocheameter Table.
First Wheel.
| No. | Fur. | Yds. | Ft. | In. | No. | Fur. | Yds. | Ft. | In. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | — | 5 | 0 | 2½ | 52 | 1 | 43 | 1 | 10 |
| 2 | — | 10 | 0 | 5 | 53 | 1 | 48 | 2 | 0½ |
| 3 | — | 15 | 0 | 7½ | 54 | 1 | 53 | 2 | 3 |
| 4 | — | 20 | 0 | 10 | 55 | 1 | 58 | 2 | 5½ |
| 5 | — | 25 | 1 | 0½ | 56 | 1 | 63 | 2 | 8 |
| 6 | — | 30 | 1 | 3 | 57 | 1 | 68 | 2 | 10½ |
| 7 | — | 35 | 1 | 5½ | 58 | 1 | 74 | 0 | 1 |
| 8 | — | 40 | 1 | 8 | 59 | 1 | 79 | 0 | 3½ |
| 9 | — | 45 | 1 | 10½ | 60 | 1 | 84 | 0 | 6 |
| 10 | — | 50 | 2 | 1 | 61 | 1 | 89 | 0 | 8½ |
| 11 | — | 55 | 2 | 3½ | 62 | 1 | 94 | 0 | 11 |
| 12 | — | 60 | 2 | 6 | 63 | 1 | 99 | 1 | 1½ |
| 13 | — | 65 | 2 | 8½ | 64 | 1 | 104 | 1 | 4 |
| 14 | — | 70 | 2 | 11 | 65 | 1 | 109 | 1 | 6½ |
| 15 | — | 76 | 0 | 1½ | 66 | 1 | 114 | 1 | 9 |
| 16 | — | 81 | 0 | 4 | 67 | 1 | 119 | 1 | 11½ |
| 17 | — | 86 | 0 | 6½ | 68 | 1 | 124 | 2 | 2 |
| 18 | — | 91 | 0 | 9 | 69 | 1 | 129 | 2 | 4½ |
| 19 | — | 96 | 0 | 11½ | 70 | 1 | 134 | 2 | 7 |
| 20 | — | 101 | 1 | 2 | 71 | 1 | 139 | 2 | 9½ |
| 21 | — | 106 | 1 | 4½ | 72 | 1 | 145 | 0 | 0 |
| 22 | — | 111 | 1 | 7 | 73 | 1 | 150 | 0 | 2½ |
| 23 | — | 116 | 1 | 9½ | 74 | 1 | 155 | 0 | 5 |
| 24 | — | 121 | 2 | 0 | 75 | 1 | 160 | 0 | 7½ |
| 25 | — | 126 | 2 | 2½ | 76 | 1 | 165 | 0 | 10 |
| 26 | — | 131 | 2 | 5 | 77 | 1 | 170 | 1 | 0½ |
| 27 | — | 136 | 2s | 7½ | 78 | 1 | 175 | 1 | 3 |
| 28 | 1 | 141 | 2 | 10 | 79 | 1 | 180 | 1 | 5½ |
| 29 | 1 | 147 | 0 | 0½ | 80 | 1 | 185 | 1 | 8 |
| 30 | — | 152 | 0 | 3 | 81 | 1 | 190 | 1 | 10½ |
| 31 | — | 157 | 0 | 5½ | 82 | 1 | 195 | 2 | 1 |
| 32 | — | 162 | 0 | 8 | 83 | 1 | 200 | 2 | 3½ |
| 33 | — | 167 | 0 | 10½ | 84 | 1 | 205 | 2 | 6 |
| 34 | — | 172 | 1 | 1 | 85 | 1 | 210 | 2 | 8½ |
| 35 | — | 177 | 1 | 3½ | 86 | 1 | 215 | 2 | 11 |
| 36 | — | 182 | 1 | 6 | 87 | 2 | 1 | 0 | 1½ |
| 37 | — | 187 | 1 | 8½ | 88 | 2 | 6 | 0 | 4 |
| 38 | — | 192 | 1 | 11 | 89 | 2 | 11 | 0 | 6½ |
| 39 | — | 197 | 2 | 1½ | 90 | 2 | 16 | 0 | 9 |
| 40 | — | 202 | 2 | 4 | 91 | 2 | 21 | 0 | 11½ |
| 41 | — | 207 | 2 | 6½ | 92 | 2 | 26 | 1 | 2 |
| 42 | — | 212 | 2 | 9 | 93 | 2 | 31 | 1 | 4½ |
| 43 | — | 217 | 2 | 11½ | 94 | 2 | 36 | 1 | 7 |
| 44 | 1 | 3 | 0 | 2 | 95 | 2 | 41 | 1 | 9½ |
| 45 | 1 | 8 | 0 | 4½ | 96 | 2 | 46 | 2 | 0 |
| 46 | 1 | 13 | 0 | 7 | 97 | 2 | 51 | 2 | 2½ |
| 47 | 1 | 18 | 0 | 9½ | 98 | 2 | 56 | 2 | 5 |
| 48 | 1 | 23 | 1 | 0 | 99 | 2 | 61 | 2 | 7½ |
| 49 | 1 | 28 | 1 | 2½ | 100 | 2 | 66 | 2 | 10 |
| 50 | 1 | 33 | 1 | 5 | 101 | 2 | 72 | 0 | 0½ |
| 51 | 1 | 38 | 1 | 7½ | |||||