40 
HINTS TO TRAVELLERS. 
Mark the ground immediately under the centre of the instrument by 
dropping the plummet from the centre of the tripod, in the usual manner, 
and from this measure a distance, in the direction the telescope points, 
equal to the focal length of the object-glass, added to the distance from 
the object-glass to the vertical centre of the instrument. Thus, if tho 
focal length of the object-glass was 12 inches, and the distance of the 
object-glass from the vertical centre of the instrument was 7 inches, then 
the position of the point from which to commence the measurement of 
the 100 feet would be 19 inches from the place where the plummet let 
fall from the centre of the tripod touched the ground. The telescope 
must always be set to solar focus, otherwise no accurate results can be 
obtained. 
To all distances measured in this manner a constant, equal to the focal 
length of the object-glass + the distance of the object-glass from the 
vertical centre of the instrument, must be added, otherwise there will be 
an increasing error in each distance that is measured. ( For instructions 
for using this instrument in the field, see pp. Ill to 116.) 
The Plane Table. 
The plane table is, in substance, a drawing board fixed on a tripod, 
so that lines may be drawn on it by a ruler placed so as to point to any 
object in sight. Its advantage is, that it enables a survey to be made 
without the aid of, and in less time than with other instruments. 
All its other parts are mere additions to render this operation 
more convenient, and accurate. Though the principle on which all 
plane tables are constructed is the Same, they vary considerably in 
detail. Those, for instance, used by the United States Coast Survey, and 
several of the European Governments, are very elaborate instruments, 
fitted with parallel plates and levelling screws, having also a telescope in 
the place of the ordinary sights. The plane table then becomes an 
instrument of precision, but is much more liable to sustain injury from 
accident than in its rougher form, not more so, however, than a theodolite 
or sextant. The levelling screws enable the traveller to set up his instru¬ 
ment much more expeditiously and accurately than he possibly could 
without them, and with the telescope he will be able to see distant objects 
that would otherwise be too indistinct to be made use of in the survey. 
