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                                         42)
and therefore Projection and Attraction will not account for the motion of Planets and Comets. W.J.

    The best method of fixing cross Hairs to a Telescope is thus, In the focus of the Eye Glass or that of the object Glass with any number of Eye Glasses fix an Hair fast down, on one side of the Tube only, then fix the telescope in ^ [[insertion]]or near [[/insertion]] the plane of the Horizon and view some distant part thereof, the farther the better, place the hair upon it and near the center of the glass as possible, [[strikethrough]] and [[/strikethrough]] in which situation fix it to the other side of the Tube; then turn the Telescope but just half round and try if the hair cuts the Horizon as before, if it does, then is it exactly in the center; but if it does not, it must be made so to do by repeated trials.  Mr. G. Adams Mathematical Instrument maker to the Prince of Whales in Fleet street London
To place the other cross hair at Right Angles with this, chuse some artificial object at a good distance in the horizon, as a Steeple, the Corner of a Church, house, &c. which are [[insertion]] raised [[/insertion]] perpendicularly. direct the telescope to it and place ^ [[insertion]] it [[/insertion]] in the center of the glass with the hair, (already fixed), upon the Horizon, let the hair be so placed across this as to run along the very edge of the Steeple, Church, &c.  turn the telescope just half round and place it as before; if the perpendicular hair runs along the same corner as before it is then truly placed, but if it should not it must be made to do so by repeated trials.  M.r G. Adams. as before. Smith's Optics Vol II. p.317. Art. 817 &c 
A Telescope is thus fixed to a Quadrant.  1st be sure of fixing it fast to the Quadrant in the manner & place designed.  2[n]d Hang a line & plummet at the center; view some distant object in or near the Horizon, so that the intersection of the cross hairs may fall exactly upon the object, and precisely mark where the line cuts the limb, there stick a pin, or a fine wire.  3rd Take off the line from the center & hang it upon this pin or wire. 4th Insert the Quadrant with particular care to have the telescope just at the same height ^ [[insertion]] from the ground [[/insertion]] as before.  5. Observe the same object, in the same manner as above, and if the line falls exactly over the center, the pin point is then in the true Horizon from which the divisions of the Quadrant must begin; and the telescope never altered afterwards: but if the line should not fall upon the center, as it most likely will not, the pin must be so moved in a Circle of that Radius until it does; then bisect the distance of these two places of the pin for the true Horizon or 0 degree upon the Quadrant from which the graduation of the rest must begin.  Stone's transl. of Bion on Instruments. p. 152.
Another way is to observe two succeeding Meridian Altitudes of any fixed Star, once with the face of the Quadrant towards the East, and once towards the West; the bisection of these two Altitudes will give the point of 45 degrees.  Bion p.152. last Edit. or p.155 old Edit.