OBSERVATIONS ON THE VARIATION. 
CXIX 
every direction of the ship’s head varies (all other circumstances remaining the 
same) as the dip of the needle increases or diminishes, and in a ratio of 
which it would be desirable to afford a practical exemplification, it was 
essential that the local attraction influencing the compass used in the obser- 
vations made for this purpose in different parts of the world, should be 
preserved, as far as might be possible, uniform and steady both in its direc- 
tion and in its absolute force ; in which case, and in which alone, the increase 
or decrease of its effects on the needle could be taken as the measure of its 
alteration in relative strength to the directive power of the earth’s magnetism, 
proportioned to changes in the amount of dip ; and so far as this steadiness 
of local attraction can be maintained on board a ship, it can only be with 
a compass used always in one and the same spot. 
Accordingly whilst the Hecla was fitting at Deptford, Captain Parry se- 
lected a position for a standard compass ; a three-legged copper-fastened 
stool was placed exactly amidships on the gun-room sky-light, the legs being 
properly secured by elects ; the middle of the stool was bored to receive the 
central pivot of Walker’s azimuth compass-box; the height at which the 
compass was thus supported was as great as the convenience of an observer 
standing on the sky-light would admit, being dy feet, or six feet two inches 
altogether above the deck ; this elevation was considered as advantageous, 
not only as it gave the compass a greater command over objects to which it 
could be directed, but also as it diminished the liability to occasional dis- 
turbance from implements and other moveable iron, which were necessarily 
kept on deck, and which were thus brought so far beneath the horizontal 
suspension of the needle, as to have their influence greatly lessened when not 
altogether destroyed. 
As an additional precaution, much care was taken whilst the ship was 
fitting, to substitute copper instead of iron work and fastenings, to as great 
an extent around the position of the compass as could be done with pro- 
priety. And at Captain Parry’s request, brass guns were supplied for the 
after-part of the Hecla’s deck, instead of the iron ones, which were part of 
her equipment. 
On the 4th of May the fitting of the ship being completed, her stores em- 
barked and distributed according to the disposition in which it was designed 
they should remain, a course of experiments were made at Northfleet in the 
River Thames, to ascertain the effects produced by the iron she had on 
board on the standard compass. 
