VS4t cook's voyage to no v. 



tion. It will also enable Mr. Dun to correct his new 

 Variation Chart, a thing very much wanted. 



It seems strange to me, that the advocates for the 

 variation should not agree amongst themselves. We 

 find one # of them telling us, as I have already ob- 

 served, that with 8 west variation, or any thing 

 above that, you may venture to sail by the Cape de 

 Verde Islands, by night or day, being well assured, 

 with that variation, that you are to the eastward of 

 them. Another, in his chart t, lays down this varia- 

 tion ninety leagues to the westward of them. Such 

 a disagreement as this, is a strong proof of the uncer- 

 tainty of both. However, I have no doubt, the former 

 found here, as well as in other places, the varia- 

 tion he mentions. But he should have considered, 

 that at sea, nay even on land, the results of the most 

 accurate observations will not always be the same. 

 Different compasses will give different variations ; 

 and even the same compass will differ from itself two 

 degrees, without our being able to discover, much 

 Jess to remove, the cause. 



Whoever imagines he can find the variation within 

 a degree, will very often see himself much deceived. 

 For, besides the imperfection which may be in the 

 construction of the instrument, or in the power of 

 the needle, it is certain that the motion of the ship, 

 or attraction of the ironwork, or some other cause 

 not yet discovered, will frequently occasion far 

 greater errors than this. That the variation may be 

 found, with a share of accuracy more than sufficient 

 to determine the ship's course, is allowed ; but that 

 it can be found so exactly as to fix the longitude 

 within a degree, or sixty miles, I absolutely deny. 



* Nichelson. . f Mr. Dun. 



