2Q'4" PHILOSOPHICAL TRANSACTIONS. [aNNO 1772. 



each other, and to the plane of the quadrant. If they were put at such a dis- 

 tance as to divide the diameter of the field of view into 3 equal parts, it micj-ht be 

 as convenient as any other Interval. In this manner wires were placed in the 

 telescope by Mr. Hadley, as appears by his account of the instrument in Philos. 

 Trans., N° 420. These wires are to be adjusted parallel to the plane of the 

 quadrant, by turning the eye tube round about which contains the wires, till 

 they appear parallel to the plane of the quadrant. The axis of the telescope, by 

 which is meant the line joining the centre of the object glass and the middle 

 point between the two wires, is to be adjusted parallel to the plane of the qua- 

 drant, by either of the two following methods. 



1st Method. When the distance of the moon from the sun is greater than 

 90 degrees, by giving a sweep with the quadrant, and moving the index, bring 

 the nearest limbs to touch each other at the wire nearest the plane of the qua- 

 drant. Then, the index remaining unmoved, make the like observation at the 

 wire farthest from the plane of the quadrant; and note whether the nearest limbs 

 are in contact as they were at the other wire; if they are, the axis of the tele- 

 scope is parallel to the plane of the quadrant: but if they are not, it is inclined 

 to the same, and must be corrected as follows. If the nearest limbs of the sun 

 and moon seem to lap over each other at the wire farthest from the plane of the 

 quadrant, the object ends of the telescope is inclined from the plane of the qua- 

 drant, and must be altered by the adjustment made for that purpose, but, if the 

 nearest limbs of the sun and moon do not come to touch each other at the wire 

 farthest from the plane of the quadrant, the object end of the telescope is inclined 

 towards the plane of the quadrant, and must be altered by the adjustment ac- 

 cordingly. Let these operations be repeated till the observation is the same at 

 both the parallel wires, and the axis of the telescope will be adjusted parallel to 

 the plane of the quadrant. In like manner, the axis of the telescope may be also 

 adjusted parallel to the plane of the quadrant for the fore observation. 



Second method. Set the index to O, and hold the plane of the quadrant 

 parallel to the horizon of the sea, with the divided arch upwards, the two wires 

 being parallel to, and including both the direct fore horizon, and the reflected 

 back horizon, between them. Raise or lower the plane of the quadrant till 

 the direct and reflected horizons coincide together: if the coincidence happens 

 in the middle between the two wires, or rather, to be more exact, above 

 the middle by such a part of the field of view as answers to the number of 

 minutes in the depression of the horizon (which may be easily estiinated if the 

 angular interval of the wires be first found by experiment, in the manner here- 

 after mentioned) the axis of the telescope is parallel to the plane of the quadrant; 

 but if it does not, the line of sight is inclined to the plane of the quadrant, and 

 must be corrected as follows. If the direct and reflected horizons, when they coin- 



