622 PHILOSOPHICAL TRANSACTIONS. [aNNO 1733. 



its motions, as not only to stop tliose vibrations, but also to hinder its being 

 thrown backwards and forwards in the tube by any shocks of the instrument; 

 and yet, as far as has been observed, will not prevent its settling, with suffi- 

 cient truth, though slowly, to the lowest part of the tube. About half a mi- 

 nute of time or more may be necessary for this, according as the aforesaid 

 sinall hole is greater or less, in proportion to the bore of the tube. When you 

 judge the spirit quite settled, turn the stop-cock again: it is of no importance 

 that the image of the object be exactly on the thread at the instant that this is 

 done. Observe against what degree, and part of a degree, eacii end of the 

 spirit ill the tube stands. If the scale be numbered like the upper one in the 

 figure, and the quantity of spirit be exact, both ends will agree, and the degree 

 and parts marked must be added to, or subtracted from, the altitude shown by 

 the index, according to the directions. If the ends do not exactly agree, take 

 the mean between them. If you use the under scale, subtract the less number 

 from the greater, and add or subtract the excess, the number resulting will 

 show the mean elevation of the index during the latter part of the observation, 

 and will differ from the true altitude of the object about half as much as the 

 vibrations of its image above and below the aforementioned thread on the vane 

 fail of compensating one another during that time. If either end of the spirit 

 leave the scale, the index must be removed 3 or 4 degrees, and the observation 

 repeated. 



Instead of the curve tubes a and b, two straight ones might be used, set 

 together so as to make a very obtuse angle in the middle; but then it will be 

 convenient to have the quantity of spirit more exactly fitted to the scale, be- 

 cause the allowing for the difference will be something more troublesome. 



If the observer have an assistant to attend to the level, while he himself ob- 

 serves the object, the whole apparatus of the brass tube and stop-cock may be 

 omitted, substituting in its stead only a plug with a small hole in it, which may 

 be wrapped round witli a very thin slice of cork, and so thrust down into the 

 middle of the glass tube. The cutting the glass tube in half in the middle may 

 likewise be avoided, if, instead of the stop-cock at g, there be one fixed in one 

 or both of the pipes i and k, to open and stop the passage of the air, having a 

 larger hole in their keys, there being also a plug, with a small hole, thrust 

 down into the middle of the tube, as before. 



The bore of the small pipes i and k, and the tube h, must not be so narrow 

 as to make it difficult to reduce the spirit into its place, if by any accident either 

 end of it should get into them. 



Mr. Hadley has been informed, that an object may be kept in view without 

 much difficulty, even in pretty rough weather, through a telescope magnifying 



