THE CAMPYLOMETER. 751 



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 centimetres shown by the upper graduation the complement in 

 millimetres furnished by the division to the Ath. 



For example, suppose we read 20 upon the upper scale, 35 the division 

 to the TrV opposite the " reglet"; the length obtained is 20 centimetres plus 

 35 millimetres, or 0*235 metres. If we are measuring upon a map on the 

 3 00*000 scale, the upper graduations represent kilometres, the complementary 

 divisions on the -V scale hundredths of metres. 



For example, suppose again 20 be the superior graduation. 35 the 

 division to the -gV of the disc as before ; the distance measured is 20 kilo- 

 metres plus 3,500 metres, or 23,500 metres. 



On the map the lower graduation of the " reglet " is used. For 

 instance, if 12 be the upper graduation of the division to the TU of the 

 distance opposite to the " reglet," the distance measured will be 1 2,700 metres. 



The campylometer has been specially constructed for maps on the 

 eoooo and uroWrr 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 millimetres 

 by the denominator of the scale divided by 1,000. 



So upon an English map to the 3-53^17, a length of 1 5 5 millimetres 

 corresponds to a " natural" length of 63,360 multiplied by 155, or 982O'8o 

 metres. 



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 : 



Having traced with the disc the line you wish to measure, carry the 

 instrument to the zero of the scale and let it run inversely the length of 

 that scale, until the zero of the disc returns opposite to the zero of the 

 " reglet." The point at which the disc is stopped on the scale indicates 

 the length of the line measured upon the map. If the scale be smaller 

 than the line measured, repeat the operation as many times as may be 

 necessary. 



If it is desirable to ascertain upon a map of a scale of 20000 the dis- 



' tance represented by 1,200 metres, we have only to place the toothed disc 



" so that its position marks four times the required distance that is, 4,800 



. metres on the map of aoooo (for 4 times 20 = 80). Then move the disc in 



the given direction until the zero returns opposite the zero of the "reglet"; 



this limit will mark the extremity of the length required. 



Explanations are not easy upon paper, but the instrument is found very 

 easy in actual use. It is employed by the military staff for calculating dis- 

 tances of any kind, curves or straight lines. On the march, or even on 

 horseback, the campylometer can be employed. 



