TOL. XXXVIII.] PHILOSOPHICAL TRANSACTIONS. ()2 1 



2 degrees are numbered as one; the reason of which is, that the motion of the 

 spirit in the tube increasing the number on one hand, and at ihe same time as 

 much diminishing that on the other, their difference is thus altered, so as to 

 answer to double that motion. The divisions of the scales are cut on the edge 

 of the brass half tube, or trough, which is made thick for the greater strength. 



In one of the small pipes i or k, just against the return of it, which enters 

 the end of the first mentioned glass tube at a or b, is a small hole, by which 

 to introduce into it so much spirit of wine as may fill it from the middle of the 

 scale on one hand, to the middle of that on the other; this hole may be after- 

 wards stopped by a screw-pin. 



The inner ends of the two halves of the glass tube ab, should be fixed into 

 the entire part of the brass tube df, with a cement made with old hard bees- 

 wax, or some other materials not dissolvable by spirit of wine; as should also 

 the ends of the small pipes i and k into this and the tube h; those halves, as 

 to the remaining part of their lengths, may be fastened down with any strong 

 cement. 



This level may be set on to one of the limbs of the quadrant, fitted up for 

 this purpose, in the manner expressed in the figure. It has an index move- 

 able on the centre, and a spring at the other end to keep it steady, when it is 

 directed to any of the divisions on the arch; which needs no other division than 

 into whole degrees. The index may be furnished either with plain sights, or 

 may carry a short telescope, with a vane in its focus, to receive the image of 

 the sun, when it is bright enough; but if the sun be hazy, or the moon, 

 or a star be observed, a sliding shutter may be drawn out to transmit the rays 

 of light to the eye-glass. The vane has also a thread fixed on it. perpendicular 

 to the plane of the quadrant. The whole instrument, for the easier managing 

 it, may be supported by a staff, resting with one end on the floor. 



The manner of using it is thus: holding the quadrant in a vertical position, 

 with that limb to which the level is fixed parallel to the horizon, raise the index 

 to some division of the arch, as near as you can to the true height of the ob- 

 ject; which is supposed to be near the meridian, and consequently to alter its 

 altitude but slowly: then turning the key of the stop-cock, so as to let the 

 spirit of wine pass through the small bole in it, keep the image of the object 

 as close to the thread on the vane as you can, endeavouring that its unavoidable 

 vibrations above and below the thread, may be equal, both in respect to their 

 length, and the swiftness of their motions, &c. Continue this till the spirit 

 seems quite settled to some part of the scale, and something longer. This it 

 will do slowly, but without any sensible vibrations; for the stop-cock allowing 

 it no passage but through the small hole in its key, will give such a check to 



