Few observers near a city can get good sky room, from the interference of houses, trees or blazing street lamps, and the telescope must often be moved from one site to another to reach different fields. In such case it is wise to take the very first step toward giving the telescope a local habitation by establishing a definite placement for the tripod. To this end the three legs should be firmly linked together by chains that will not stretch—leg directly to leg, and not to a common junction. Then see to it that each leg has a strong and moderately sharp metal point, and, the three points of support being thus definitely fixed, establish the old reliable point-slot-plane bearing as follows: Lay out at the site (or sites) giving the desired clear view, a circle scratched on the ground of such size that the three legs of your tripod may rest approximately on its periphery. Then lay out on the circle three points 120° apart. At each point sink a short post 12 to 18 inches long and of any convenient diameter, well tarred, and firmly set with the top levelled off quite closely horizontal. To the top of each bolt a square or round of brass or iron about half an inch thick. The whole arrangement is indicated in diagram in Fig. 171. In _a_ sink a conical depression such as is made by drilling nearly through with a 1 inch twist drill. The angle here should be a little broader than the point on the tripod leg. In _b_ have planed a V shaped groove of equally broad angle set with its axis pointing to the conical hole in _a_. Leave the surface of _c_ a horizontal plane. Now if you set a tripod leg in _a_, another in the slot at _b_ and the third on _c_, the tripod will come in every instance to the same level and orientation. So, if you set up your equatorial carefully in the first place and leave the head clamped in azimuth, you can take it in and replace it at any time still in adjustment as exact as at the start. And if it is necessary to shift from one location to another you can do it without delay still holding accurate adjustment of the polar axis to the pole, and avoiding the need of readjustment. In case the instrument has a declination circle the original set-up becomes even simpler. One has only to level the tripod, either with or without the equatorial head in place, and then to set the polar axis either vertical or horizontal, levelling the tube with it either by placing the level across the objective cell perpendicular to the declination axis, or laying it along the tube when horizontal. [Illustration: FIG. 171.—A Permanent Foothold for the Tripod.]