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<‘ See that the jolly-boat, towing under the counter, drops 
astern a hundred yards or so, and thatit contains a careful 
officer and good compass, seeing of course that the boat is free 
from iron ballast, rings, &c. Pay out the tow-line until the boat, 
as described, is a hundred yards astern. One officer is now 
in the boat ; another stands by the binnacle compass ; a third 
takes his position at the taffrail, looking towards the boat. The 
officer in the boat, and the officer at the binnacle, each keeps one 
eye, as it were, upon his respective instrument, and one upona 
little flag which the third officer, who stands at the taffrail, holds 
in his hand. The moment this look-out officer finds the jolly- 
boat’s head and stern to be on a line with the keel of the ship, 
he lowers the flag. The other two officers check their respec- 
tive compasses as the flag falls. Zhe difference between the 
ship’s compass and the boat’s compass is the error sought after. 
‘« Because, if the ship’s course was ‘ due east,’ the course of 
the boat was ‘due east’ likewise, since her head and stern 
were in a line with the ship’s head and stern by observation. 
If the course of the ship and boat were ‘due east,’ the fleur 
de lis on binnacle and boat should stand. at right angles to the 
bows of ship and boat. But the local attraction of the ship 
interferes with this result, and as the boat’s compass is the 
correct one, the difference between the instruments is the in- 
fluence of local attraction on board the ship. 
*‘ Asa first step, this is the simplest mode of escaping from 
the threatened overturning of our faith in the mariner’s com- 
pass. The other plans of ‘floating compasses’ are merely 
alterations in the form of the experiment, which do not inter- 
fere with the principle just detailed in the slightest degree. I 
give the following as the best I have devised. 
“Take a long plank of timber; cut it sharp at either end ; 
make a hole in the middle, insert a compass therein, having a 
spring so arranged that by chucking a trigger the needle is 
instantly arrested at any point. Place a shining mark at the 
head and stern of this plank. Now float it astern at the end 
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