VOL. LXII.] PHILOSOPHICAL TKANSACTIONS. 2Q5 



cicle, appear higher above the middle between the wires, than what the quantity 

 of the depression of the horizon amounts to, the object end of the telescope is 

 inclined from the plane of the quadrant, and must be altered by the adjustment 

 made for that purpose; but if the two horizons appear to coincide in a lower part 

 of the field of the telescope, the object end of the telescope is inclined towards 

 the plane of the quadrant, and must be altered by the adjustment accordingly. 

 Repeat these operations till the two horizons appear to coincide above the middle 

 between the two wires, by the quantity of the depression of the horizon, and 

 the axis of the telescope will be adjusted parallel to the plane of the quadrant. 

 In order to find the angular interval between the wires, hold the quadrant 

 perpendicular to the horizon, as in observing altitudes ; and turn about the eye 

 tube with the wires till they are parallel to, and include, the direct fore horizon 

 and reflected back horizon between them. Move the index from O along the 

 divided arch, at the same time raising or lowering the telescope by the motion 

 of the quadrant, till the direct horizon appears to coincide with the upper wire, 

 and the reflected back horizon with the lower wire ; the number of degrees and 

 minutes shown on the arch, increased by double the depression of the horizon, 

 will be the angular interval of the wires; its proportion to the depression of the 

 horizon will be therefore known ; and hence the space in the field of the telescope 

 answering to the depression of the horizon, may be easily estimated, near enough 

 for adjusting the axis of the telescope in the manner before mentioned. The 

 first of the two methods here given, for adjusting the position of the telescope, 

 will probably be found most convenient ; and the greater the distance of the sun 

 and moon is, the more nearly may the adjustment be made, because the same 

 deviation of the axis of the telescope will cause a greater error. 



The telescope should be fixed by the instrument maker, so as to command a full 

 field of view when the instrument is placed at 90° if the instrument be an octant, 

 or 120° if it be a sextant ; because the index glass then stands more oblique with 

 respect to the incident and reflected rays, and consequently the field of view of 

 the telescope, as far as it depends on the index glass, will be more contracted 

 than in any other position of the index : but if there is a fair field of view in this 

 case, there necessarily must be so in every other position of the index. 



The two parallel wires will be very useful on many occasions, as well in the 

 fore as the back observation. In taking the altitude of the sun, moon, or star, 

 direct the sight towards the part of the horizon underneath, or opposite to the 

 object, according as you intend to observe by the fore or back observation, and 

 hold the quadrant that the wires may constantly appear perpendicular to the 

 horizon, and move the index till you see the object come down towards the 

 horizon in the fore observation, or up to it in the back observation, and turn 

 the instrument in order to bring the object between the wires; then move the 



