| 1. | 5, 10, 15, 20 | 50 centi. | according to metric scale. |
| 2. | 5, 10, 15, 20 | 50 kil. | scale 1/100000; |
| 3. | 4, 8, 12, 16 | 40 kil. | 1/80000; |
The micro-metric screw is fixed in a frame so made as to form a kind of indicator or guide at one side.
To make use of the campylometer, bring the zero of the disc opposite the zero of the rule (reglet), then place the instrument on the map in a perpendicular position; the point will serve as guide, and move the disc upon the line, whether direct or sinuous, of which you wish to ascertain the length.
When this has been done, note the last graduation of the “reglet” beyond which the disc has stopped, add to the value of this graduation the complementary length shown by the division of the disc which is opposite the “reglet.” To find the length of a material line we must add to the number of centimètres shown by the upper graduation the complement in millimètres furnished by the division to the 1/50th.
For example, suppose we read 20 upon the upper scale, 35 the division to the 1/50 opposite the “reglet”; the length obtained is 20 centimètres plus 35 millimètres, or 0·235 mètres. If we are measuring upon a map on the 1/100000 scale, the upper graduations represent kilomètres, the complementary divisions on the 1/50 scale hundredths of mètres.
For example, suppose again 20 be the superior graduation, 35 the division to the 1/50 of the disc as before; the distance measured is 20 kilomètres plus 3,500 mètres, or 23,500 mètres.
On the map the lower graduation of the “reglet” is used. For instance, if 12 be the upper graduation of the division to 1/40 of the distance opposite to the “reglet,” the distance measured will be 12,700 mètres.
The campylometer has been specially constructed for maps on the 1/80000 and 1/100000 scales, and calculations can be made easily for any maps whose scales are multiples or sub-multiples of these. But the instrument will serve equally well for all maps or plans of which the numerical scale is known. We must multiply the length of the line expressed in millimètres by the denominator of the scale divided by 1,000.
So upon an English map to the 1/63360, a length of 155 millimètres corresponds to a “natural” length of 63,360 multiplied by 155, or 9820·80 mètres.
Thus we perceive that the employment of the campylometer does not necessitate the tracing of a graphic scale, but only the knowledge of the numerical scale. When the former only is known, the campylometer may be used in the following manner:—