'2,62 PHILOSOPHICAL TRANSACTIONS. [aNNO 1738. 



The Description and Use of an apparatus added as an Improvement to Davis's 

 Quadrant, consisting of a Mercurial Level, for taking the Co-altitude of the 

 Sun or a Star at Sea, wilfiout the usual Assistance of the sensible Horizon, 

 which frequently is obscured. By Charles Leigh, Gent. N°451, p. 417. 



Since the former communication, in the foregoing article, Mr. L. has made 

 such alterations and improvements in it, as have rendered it complete and per- 

 fect for the use intended, which have been confirmed by repeated experiments, 

 as well on board ships, as on shore. 



To arrive at perfection in navigation, 3 things are absolutely requisite, viz. 

 the variation, the latitude, and the longitude ; which last is as yet concealed 

 from us. The two former indeed, we have a tolerable certainty of, especially 

 the first, which may be found by observation, almost at any time the sun is 

 visible, in or above the horizon, either by an amplitude or azimuth ; but it is 

 not so in regard to the latitude, by any certain method, but what is considered 

 as too abstruse for common practice ; for it is but once in 24 hours that an ob- 

 servation can be made by the sun, and even that space of time is so very short, 

 that if the horizon should then be obscured, or a cloud intercept the sun's rays, 

 the dead reckoning is then the only guide, which in fact, is little better than 

 groping in the dark. 



The first instrument made conformable to the same principle, was with a 

 water-level ; but finding that water was subject to some inconveniencies, Mr. 

 Leigh has altered the apparatus, and changed the fluid from water to mercury : 

 this alteration and improvement will better appear by the instrument, repre- 

 sented fig. 3, pi. 7, where ab, cd, represent the segments of two different 

 concentric circles; e the common centre, in which moves the pin or axis fitted 

 to the index or label ef ; on which label is also fixed the horizontal tube Gg, 

 which has a communication with the two glass vertical tubes Eh, dh, in which 

 the mercury moves. On each top of the vertical tubes are fixed a large hollow 

 brass cylinder hh, having in their tops a pin, by closing of which, the included 

 air is prevented from any communication with the external ; by which means 

 this advantage is obtained, that it prevents, in a great measure, that too quick 

 and vibratory motion, natural to the fluidity, joined to the gravity of mercury 

 when moved, and at the same time, by having a sufficient space and quantity of 

 air in the cylinders at top, does not in the least impede the true level ; but not- 

 withstanding this precaution, the mercury still would be subject to a tremulous 

 motion, were it not that the diameters of the vertical tubes, to that of the 

 horizontal, are as 2 to 1, and consequently the area 4 to I ; by which means 



