VOL, XXXIir.] PHILOSOPHICAL TRANSACTIONS. Q5 



graduated each way, and a few of the degrees, about that part of it which 

 answered to the dip, were divided into 6 equal parts. By means of screws, 

 the semicircle could be brought to a due situation ; and by two spirit levels, 

 placed at right angles to each other, any change of situation was easily per- 

 ceived, and by the screws it could be readily restored to its former position ; 

 all was inclosed with glass to secure the needle from being disturbed by the 

 motion of the air. He notices the great difficulty of poising the needle so 

 exactly, before it is touched with the loadstone, as to take any position indif- 

 ferently: for, when it is pretty near the truth, it is extremely troublesome to 

 place it at rest in the position desired, in order to try which way it is inclined 

 to move. It cannot be done in the open air; for the least motion of it will 

 disturb the needle; and when it is shut up, it is no easy matter to settle it in 

 the place intended. And that there will be a sensible difference of the dip, on 

 shifting the sides of the needle, whatever pains be taken to prevent it, fully 

 appears from the following experiments. 



Exper. ]. March 20, 1722. Mr. G. touched both sides of that end of the 

 needle, which intends to point south, on the north-pole of a small terrella; 

 after which he caused it to vibrate in an arch of 10 degrees, and counted the 

 time by a pendulum clock, showing seconds, till the needle had performed 50 

 vibrations. 



It performed the first 25 vibrations in 2"" 58"; the next 25 vibrations in 

 qm 27s . the 50 in 5™ 25*; which gives for each vibration at a medium 6\5. 

 The needle dipped 73° 15'. 



Exper. 1. He then shifted the needle, so as that side which before respected 

 the east, was now turned west, and causing it to vibrate in the same arch, as before. 



It performed the first 25 vibrations in 2" 49' ; the next 23 in 2™ 39'; the 

 50 vibrations in 5"" 28^; that is, each vibration in 6\56. The dip was 73° 50'. 



Exper. 3. He now touched the same end of the needle, a second time, on 

 both sides, on the same stone, and suffering it to vibrate, as before, 



It performed 25 vibrations in 2"" 49* ; that is, 1 vibration in Q'.yQ. The 

 dip being 73° 20 . 



Exper. 4. The needle was now shifted, and stood as in the 2d experiment. 



It performed 25 vibrations in 2"* 41"; that is, 1 vibration in 6^44. The dip 

 being 73° 45'. 



Exper. 5. The same end of the needle being now touched twice on eacli 

 side, with the loadstone presented by Lord Paisley to the Royal Society, in 

 the armour. 



It performed the first 25 vibrations in 1™ 58*; the next 25 in 1"' 40'; the 50 

 vibrations in S'" 44'; that is, each vibration in 4'.48. The dip being 73° 65'. 



