310 I'HILOSOPHICAL TRANSACTIONS. [aNNO 1793. 



from sonic deviation in the centre. And since the y's hanging in glmmals was a 

 new experiment, this error was supposed to take its rise from some shake in them. 

 They were examined ; and were altered in various ways. Fixed y's were then 

 made, of the usual form ; others of a larger; others of a more acute angle. The 

 difficulty was still thought to continue. Recourse was then had to y's in gim- 

 mals again, which I was unwilling to give up ; and friction-rollers were applied to 

 take off some of the weight. Still this error did continue in a small degree : yet 

 was that degree so small, as not to be discernible at the polar microscope ; nor, 

 as far as I could see, at those belonging to the plumb-line ; and sometimes scarce- 

 ly so at the others, to whose greater magnifying power it seemed to be owing 

 that it was at all perceptible. The cause I then supposed to be, in a disposition 

 in the pivots to gather up the side of the y's towards which they were turned. Yet 

 that was not the cause : for what little motion there was, I found afterwards to be 

 in a contrary direction. 



This led me into discovering, and at last rectifying the defect. The original 

 idea of hanging the y's in gimmals, as was said before, was derived from Mr. 

 Smeaton ; who kindly showed to Mr. Cary those which he had made to a small 

 transit instrument for his own use. His ought scarcely, in strictness, to be called 

 y's ; for he had made a little hollow on each side where the pivots would touch, 

 as a sort of bed to receive them, and make the angle less pinching. This Mr. 

 Cary had imitated : and though I did not mean he should, he did the same to the 

 2d pair he made, after trying the other kinds. Since it was done, I let them so 

 remain till I got the instrument home ; for I really found all trials so disturbed 

 by the shaking of carriages while it was at his house, that I could make no satis- 

 factory examination there myself. When the instrument was in its place, I tried 

 every experiment I could contrive to discover the cause of this error ; whether it 

 could be in the microscopes themselves ; any shake in them, or in the pillars, or 

 in the hanging of the y's. Finding none of these to be in fault ; and, on try- 

 ing the instrument at every 10' all round, perceiving the axis thrown backward 

 instead of forward on turning either way, it occurred that any grease or other 

 particles would have it more in their power to produce that effect in a sort of 

 pivot-hole, which the hollowed sides really are, than between 2 fair flat surfaces. 

 I therefore took out the y's, and had them formed to an exact right-angle, with 

 the whole sides perfectly smooth, and flat, and well finished : and since that has 

 been done, I really can discover no difference which ever way the circle is turned; 

 but think I may now say that deviation is quite removed. 



Yet I apprehend it would have been of no consequence if it had continued, or 

 been greater than it was. For since the readings are as it were in a line above 

 and below the centre, and both of them positive ; any motion of the centre to- 

 wards the right hand, would give the dots, both above and below, the appear- 



