THEORY AND PRACTICE OF ENGINEERING.

Book 11.

SS2

To make Surveys under Ground , as in Mines ,tri'M the Compass .— Supposing it required tomake an opening at A, and after a shaft has beensunk to the required depth, to cut a road in themine which shall continue in the direction of andterminate at a point directly under B : we mustplace ourselves above the opening A, as at C, andthen measure in a right line the distance fromA to B, which in this case is 28 feet; then placethe north side of the compass at the point C, onthe line CB, and observe at what degree theneedle points, as to the 7° E. of N., and figurethis down : then having removed the compass,draw on the ground at the point C, and on theline C B, the perpendicular C G, and lay a staffalong this line C G, which carries at its salientextremity a plumb-line, to mark the point II onthe ground at A, and also several others, as I K,&c., drawing back the rod on the line C G ; takecare to observe how much the rod was advancedfrom the point C, where it marked the pointH, as 9 feet: then from the entrance A, draw' aright line through the points H, I, K, measuringback 9 feet from H, which will give the pointR, answering to the point C above ground:place the side of the compass marked north atthe point R, and turn the compass till the needlestops 7° E. of N., which being remarked, drawa line along the north side as R S, in order todig according to this line from the point R 28feet, which will coincide with the point B ; butshould it not be practicable to continue in astraight line on account of the hardness of therock, or for some other cause, several detours atright angles may be made to the right and left,as shown at O, P, &c., taking care that thenumber of small detours, P, compensate for thedetour O, to find again at V the right line It S.

It is by the aid of geometry that the minerstudies the situation of the mineral deposits onthe surface and in the interior of the ground : healso determines the several relations of the veinsand rocks, and becomes capable of directing allthe operations which are to be performed.Wherever iron does not interfere with the mag-netic needle, the compass is employed tomeasure the direction of a metallic vein, andits graduated circle serves to notice the in-clination or dip, which is called the clinometer.The distance from one point to another is mea-sured with a staff or chain ; for some surveysthe dials of the compasses are graduated intohours, generally twice twelve ; thus the wholelimb is divided into 24 spaces, each of whichcontains fifteen degrees or one hour, which issubdivided into eight parts.

The Jacob's Staff, an instrument used by geo-metricians to measure distances and heights, isa long rod or square staff with a cross, as shownat D E ; it is usually made of the wood of the wildcherry, pear tree, or ebony, these three treeshaving fewer knots or veins, which allows the cur-sor to move freely up and down the rod : the roditself is divided usually into four equal parts, as

Fig.1045.

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Fig. 1046.

at M, or five as at L, and is commonly 3 or 4 feet in length : the cursor has a square holemade in the middle, and is allowed to have but tittle play ; it is provided with a smallscrew to keep it at any required point. In order to measure distances, it should be in