VOL. XL.] PHILOSOPHICAL TRANSACTIONS. 26l 



section cd, on which index is fixed a brass tube 15 inches long, in the extremi- 

 ties of which are fixed perpendicularly two glass tubes Eh and dh. 4 inches long, 

 with brass ferrels on the tops. 



On the central pin, which is fixed in the index, is also fixed the brass hori- 

 zontal vane ez obliquely, in which is a hole for the central glass tube Eh, to come 

 through three fourths of its length; close to which, and from the common 

 centre, comes a white fine thread, the end being fixed in the vane ez ; and in 

 the same manner is a thread fixed close to the glass tube dh. 



To prepare this instrument for observation, pour water into the tube sh, till 

 its little surface rises to the central thread ; then to keep it fixed there, shut the 

 slide or stop, fixed on the top of the central tube, and there it will continue ; 

 then you may at pleasure pour or drop water into the tube dh, till its surface 

 also rises to the thread fixed there; and if too much water is dropped in, dip in 

 a wire with a small bit of sponge or cotton fixed to -the end, till you exactly 

 trim the tubes; for the greatest nicety and exactness lies in trimniing the sur- 

 faces true to the threads. 



Being thus prepared for observation, place yourself conveniently, where there 

 is the least motion, on a stool or the deck, and having the quadrant in its pro- 

 per position on your lap, open the slide on the top of the tube eh, that the 

 water may have its natural tendency, which will be truly horizontal, conforma- 

 ble to the above principle ; then keeping your eye on the central thread, bring 

 that and the little surface into one, which will be effected with the same ease, 

 as if you observed by the natural horizon ; then keep moving the end of the 

 index f, till you bring the speculum of the sun in the little hole on the horizon 

 vane, close to the thread, so that you have, as it were, but one object to look 

 at during the time of observation. 



But if you use the shadow vane, you must bring the upper edge of the 

 shadow on the central line, drawn on the horizon vane, as usual ; remember- 

 ing as often as you rest, waiting the sun's rising, to close the slide, which pre- 

 vents the water's running out, it then remaining immoveable. And thus con- 

 tinuing to do, till the sun is on your meridian, cast up the two sums as is usual, 

 that is, the degrees cut by the shadow vane, and those cut by the upper edge 

 of the index on the greater arch, which sum will give what is required, viz. the 

 sun's distance from the zenith. On the end of the index is fixed a sight vane n. by 

 which you may observe by the natural horizon, the very same way as with the 

 common quadrant ; so that the one will be the proof of the other. 



