and an Account of Observations made with it. 271 



luminaries are shining bright above, that from haze, fog, or 

 darkness, the horizon cannot be seen, and the quadrant is 

 therefore useless to the mariner, who must consequently for 

 the time be left without any guide but his reckoning, how- 

 ever perilous his situation may be. How many brave hearts 

 have quailed with anxiety in entering the Channel under such 

 circumstances ! 



I have lately had put into my hands an instrument called 

 the Hydrostatic Quadrant, recently invented by the Rev. 

 Wm. Barclay, minister of Auldearn, by Nairn, in Scotland, 

 which appears more likely to effect this desirable object than 

 any other that has been devised for the purpose, of which I 

 have seen any account. 



Mr. Barclay, I understand, has taken out a patent for his 

 invention ; and this instrument (the only one that has yet been 

 made) was entrusted to me, that by an extensive and varied 

 series of observations with it on shore, I might determine what 

 degree of accuracy might be attained by the use of it, as well 

 as form some judgement respecting the probability of its being 

 useful at sea. 



. I have accordingly made a very extensive series of obser- 

 vations : but before I give the results, it may be well to state 

 the principles on which the instrument is constructed, and to 

 give a short description of it as at present fitted up for use : 

 and I may at once say, that should the instrument be found 

 to realize the hopes of its inventor, I have no doubt whatever 

 that the ingenuity of British artists will soon greatly simplify 

 its construction. 



With respect to the principle on which its operation de- 

 pends, — if we conceive a circular tube, as AEBD (Plate III. 

 fig. 1.), open all round, to be partly filled with a fluid, then it 

 is obvious that the fluid will always occupy the lower part of 

 the tube as ADB, and the line AFB joining the tops of the 

 fluid will always be horizontal; and if DFE represent the 

 axis of a telescope parallel to the plane of the tube, and pointed 

 to any object, then, if the plane of the tube be vertical, the 

 angle EFB, which is measured by half the sum of the arcs 

 AD and EB, will be the altitude of the object. 



But the position of the line AB will vary with every change 

 in the position of the instrument; so that supposing the tele- 

 scope pointed to an object, it becomes a question, — How are 

 the lengths of the arcs AD and EB to be determined at that 

 instant ? This Mr. Barclay enables us to do by a very in- 

 genious device : he inserts a stop-cock, as at C, in the tube, 

 which being turned at the instant the observation is made, both 

 the fluid and the air in the tube become fixed in their posi- 

 tions, 



