632 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



year taken when the instrument was at Rook's Hill, and which were now wanted 

 by the surveyors of the ordnance. 



As the best method of obtaining the direction of the meridian, is by observ- 

 ing the pole star on each side of the pole, whence the double azimuth is nearly 

 obtained without any correction for the star's apparent motions, every opportu- 

 nity was watched, of observing it at the times of its greatest apparent eastern 

 and western elongations. But in the unsettled season of the month of April, 

 when almost every wind brought a fog over the station, many days elapsed 

 without our seeing either the star or staff". As the truth of the deductions must 

 entirely depend on the accurate determination of the directions of the meri- 

 dians, the greatest care was taken in making the observations. An hour, and 

 generally more, before the star came to its greatest elongation, the observers 

 repaired to the tent for the purpose of getting the instrument ready. The me- 

 thod of adjusting it, was first by levelling it in the common way with the spirit 

 level which hangs on the brass pins; and afterwards, by that which applies to 

 the axis of the transit. The criterion which determined the instrument to be 

 properly adjusted, was the bubble of the latter level remaining immoveable be- 

 tween its indexes, while the circle was turned round the axis. 



As the star, 4 minutes either before or after its greatest elongation, moves 

 only about a second in azimuth, the time was shown sufficiently near, by a good 

 pocket watch, which was regulated as often as opportunities oflered. When 

 the star was supposed to be at its greatest elongation, the observer, if at night, 

 brought it on the cross wires, and bisected it, leaving equal portions of light on 

 each side of the cross; but if it was in the day, when the star appeared like a 

 point, the telescope was moved in the vertical till it came near the vanishing 

 point of the cross. At either of these times, when the observer was satisfied of 

 the star being properly bisected, or brought into the vanishing point formed by 

 the wires, another person who had kept his eye at the microscope, bisected the 

 dot. The transit was then taken oft", and the instrument being turned half 

 round, and the telescope replaced, the star was observed again. This precaution 

 was taken to obviate the errors which might arise, from the arms of the instru- 

 ment being out of the parallel with the plane of the circle, owing to any imper- 

 fections in the position of the ys, on which the transit rested. A mean of the 

 readings was always taken. 



After the business was finished at Dunnose, the instrument was taken to 

 Chanctonbury Ring, and Ditchling Beacon; and from the latter place to Beachy 

 Head, in order to observe the direction of the meridian; but after placing a staff 

 on the high ground above Jevington, we were obliged to defer the attempt, as it 

 was found, that owing to the effects of heat, the air was not sufficiently steady 



