Compass, is an instrument of considerable utility for surveying , dialling, etc.. Its structure varies but little from that of the mariners' compass ; for, instead of needle being fitted into the card and playing on a pivot, in thus instrument it plays alone; the card being drawn on the bottom of the box, and a circle divided into. 360 degrees on the limb. It is particularly useful to travellers, to direct them in their road ; also to miners, whom it guides in digging; and may be applied to various other purposes.
To take the declination of a wall by the compass : - Apply that side of the instrument, on which the north is delineated, along the side of the wall; the number of degrees, over which the north end of the needle fixes, will be the declination of the wall on that side. For instance, if the north of the needle point towards the north, the sun may shine on that wall at noon: if it fix over 50 degrees, computed from the north towards the east, the declination will be so many'de-grees towards the east.
Bat, as the needle itself, in Britain, declines 13 degrees from the north towards the west, the same number of degrees should always be added to those pointed out by the needle, when the declination of the wall is towards the east, in order to retrieve the irregularity: on the contrary, when the declination is towards the west, it will be necessary to subtract the above-mentioned declination of the needle.