4. 
VOYAGE FOR THE DISCOVERY 
1819. ours, owing to the distance at which we passed it. There is, perhaps, no 
more striking proof of the infinite value of chronometers at sea, than the 
certainty with which a ship may sail directly for a single rock like this, rising 
like a speck out of the ocean, and at the distance of forty-seven leagues from 
any other land. At seven P.M., the Griper having again dropped five or six 
miles astern, we hove to for her to come up ; and, taking this opportunity to 
try the temperature of the water below the surface by Six’s self-registering 
thermometer, we unexpectedly obtained soundings in one hundred and 
forty fathoms, on a bottom of very fine white sand, Rockall bearing S. 85° 
E., distant thirty miles and three-quarters. The temperature of the water at 
the bottom was that of the surface being 4Q^°, and of the air 50°. The 
Griper was again taken in tow, with a breeze from the eastward, which in- 
Tues. 25. creased to a fresh gale the following morning, when the hawser, by which 
we towed the Griper, gave way ; we hove to for her in the evening, being 
in lat. 57° 04' 10", long. 17° 52' 50", when some water was brought up from 
one hundred fathoms’ depth in the bottle contrived by Doctor Marcet ; its 
specific gravity was 1.0268, at the temperature of 58°, that of the surface 
water being the same. The temperature of the water at the same depth was 
49°, that of the surface being 50°, and of the air 50|°. 
Thur. 27. On the 27th, we cast olf the Griper, and hauled a little to the northward, 
in order to pass near the spot where Lieutenant Pickersgill obtained sound- 
ings, from three hundred and twenty to three hundred and thirty fathoms, 
on the 29th of June, 1776; and, at six P.M., being in lat. 56° 59' 39", and 
long, by chronometers, 24° 33' 40", the deep-sea clamms were sent down 
with one thousand and twenty fathoms of line, without finding bottom. 
The temperature of the sea at that depth was 45^°, that of the surface being 
48i°, and of the air 49°. 
Frid. 28. It fell Calm towards noon on the 28th, the ship being in lat. 57° 26' 16", 
long. 25° 11' 51". The current was tried in a boat moored by an iron kettle, 
in the usual way, but not the smallest stream was perceptible. Six’s ther- 
mometer was sent down to one hundred and twenty fathoms, but did not 
indicate the temperature, owing to the mercury rising past the index, instead 
of pushing it up before it ; a failure I have often had occasion to regret in 
this useful instrument, when thus exposed to a very sudden change of 
temperature. It might, perhaps, be improved for this particular purpose, 
by making the lower end of each index a little larger, so as to prevent the 
passage of the mercury between it and the tube. Some water, from one 
