chronometers by the proximity of mass m of iron. 381 
to be that uniformity of relation in these cases, that we should 
naturally have anticipated. 
As a practical conclusion, it is obvious, that on ship-board, 
great care ought to be taken to keep the chronometers out 
of the immediate vicinity of any considerable mass, or surface 
of iron ; on which account, they ought not to be kept in the 
cabins of the gun-room officers, which are on the sides of the 
vessel ; and probably a strong iron knee, or even a gun, will 
be found at a very inconsiderable distance from the spot where 
the watch is most likely, in this case, to be deposited. 
In short, it appears from the preceding experiments, that 
a chronometer ought to be kept as carefully at a distance 
from any partial mass of iron, as the compass itself. And, 
as much of the iron of a ship is hidden, the best way of de- 
tecting it, and of ascertaining a proper situation for a chro- 
nometer, will be to set down a compass in any place designed 
for the former, and to observe and compare the direction of its 
needle with that of the azimuth compass on deck, while the 
vessel is on different tacks ; and if the disagreement between 
the two be very considerable, another situation ought to be 
selected. 
When I made my experiments on local attraction, on board 
His Majesty's ship Leven, we placed several compasses in 
different parts of the vessel, some of which were very power- 
fully affected under different directions of the ship's head ; 
in consequence, no doubt, of their being within the influence 
of partial action arising from some near, but hidden, mass of 
iron. 
In support, and in confirmation of the necessity of taking 
the above precautions, it may not be amiss to state the fol- 
