372 A Method to correct the Variation of the Compass. 



ally indicate her course, without the necessity of a reference 

 to the last observation ; and at the same time serve as a 

 check upon the several courses entered from the steering- 

 compass. 



Whatever obstacles may exist to the application of this 

 contrivance to the ship's compass, I apprehend that there 

 will be none to iis utility, as applied to the instruments used 

 in land surveying ; confined as that business is to tracts of 

 country within which only a trifling annual variation takes 

 place, and to which variation the corrector may be set 

 ■whenever it becomes considerable. 



To the common boat- and pocket- compass the same prin- 

 ciple may be applied with equal utility ; to the latter espe- 

 cially, which may frequently, perhaps, be in the hands of 

 persons unacquainted with either the bearing or amount of 

 the variation of the magnetic needle. 



As the instruments submitted to the Committee are 

 merely intended for illustration, T have not aimed at great 

 nicety in their mechanism. It is obvious that they are 

 capable of considerable improvement, and that both the 

 theodolite and the ship's compass may be so constructed 

 as to be capable of all requisite accuracy in their adjust- 

 ment, without loading them in such a manner as to obstruct 

 the free motion of the needle. 



If a modification of these instruments, in itself so sim- 

 ple, shall be found to be of the least utility, I shall feel 

 highly gratified. 



I am, sir, 



Your very obedient servant, 



Charles Street, St. James's Square, JoHN HODGSON. 



April 23, 1811. 



To C. Taylor, M.D. Sec. 



Reference to the Draiving of Mr. John Hodgson's Vari' 

 at'ion Compass. Plate IX. fig. 1 and 2. 

 The two figures in this plate are, a plan and perspective 

 view of a small theodolite or cirrumferentor, for taking 

 horizontal angles by the magnetic needle; it is to the latter 

 alone that Mr. Hodgfon's improvement applies; being a 

 brass needle a, fig. 1, which is fitted upon the brass centre- 

 piece, or cap, of the real needle b, in such a manner that 

 it can he easily turned round to make any angle with the 

 needle, and having sufficient friction to remain where it is 

 placed ; both needles are poised on the same centre-point, 

 with an agate cap in the usual nmnner, the brass needle 



being 



