382 Mr. Barlow on the effects produced in the rates of 
lowing fact. A very intelligent seaman, many years a Master 
in the Navy, and at present an officer in the Dock- yard at 
Woolwich, to whom I was describing the nature of my ex- 
periments, immediately exclaimed, that they explained a cir- 
cumstance which he had remarked when he was master of a 
first rate. He informed me, he always found that his chro- 
nometer, which was a very excellent one, had a different rate 
on board and on shore, amounting to 5" per day ; but as he 
well remembered that the birth he had selected for it was in 
his cabin, nearly in contact with an iron knee, he now saw 
that it was the action of that mass of iron which had caused 
all his perplexity. 
Lastly ; since it is rendered obvious by the experiments 
with the plate of iron on Nos. IV. and V. that the power of 
the iron to disturb the action of the chronometer resides ( as 
in the instance of the compass), on the surface, and as we 
know, generally, the distance and direction of such a plate, 
so that its power may be equal to the mean action of the iron 
of the vessel, we have thence a ready method of ascertaining, 
before a chronometer is sent on board, whether the effect of 
the ship's iron will be to accelerate or retard its going ; and 
probably, a very near approximation to the actual quantity of 
that change may also be predicted. 
For this purpose, it is only necessary to have a box or 
pedestal, as shown in Figure 3, Plate XXV., in the side of 
which a brass pin, a b, may be fixed, to carry the iron plate 
P, and on the top of the box a convenience for placing the 
chronometer. Then, having taken its rate in the usual way, 
let it be taken again while the chronometer is placed on the 
pedestal, keeping the plate, generally, at the distance of about 
