“Mr. Bush, do you see the battery?”
“You will the longboat. Mr. Rayner will take the launch, and you will land and storm the battery.”
“Aye, aye, sir.”
“I will give you the word when to hoist out.”
“Aye, aye, sir.”
“Quarter less eight,” droned the leadsman — Hornblower had listened to each cast subconsciously; now that the water was shoaling he was compelled to give half his attention up to the leadsman’s cries while still scrutinizing the battery. A bare quarter of a mile from it now; it was time to strike.
From Ship of the Line, C. S. Forester.
For a sailor, knowing the depth of the water under the keel was probably more important than knowing a precise northing and easting. You can fix being lost. Not so much with being sunk.
As I’ve explained before, I’m a soldier by profession, and inclination, and delving into naval lore of the Age of Sail has been quite an eye opening experience.
For instance, when I was a teenager and first introduced to the Hornblower novels I visualized the leadsman being somewhere out on the bowsprit or in the beakhead, the notion of “chains”, in my landlubber’s mind, was something attached to the anchors.
Typically, the leadsman stood in the main chains, though it seems that sometimes he was stationed in the fore chains, and cast a hexagonal weight attached to a length of line. As the ship passed over the weight he called off the depth. The line had markers attached so the leadsman could call the depth in darkness. The markers are located at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms
Below is a sounding lead with line from the National Maritime Museum.
In the British navy the weight of the leads were 7 pounds (small boats), 14 pounds (standard), and 28 pounds (deep sea). The weights are all based on the British unit of weight the stone, or 14 pounds.
The lighter weights typically were attached to about 25 fathoms (150 feet) of line, the deep sea lead carried about 100 fathoms of line, or in Imperial nautical measurement, a cable.
The lead had a concave base which was charged with tallow. This enabled the lead to bring up a bottom sample to compare with charts and help fix position more accurately.
As the leadsman called depths according to a formula. When the depth fell on one of the markers affixed to the line, the leadsman would call “By the mark five (or seven or whatever)”. If the depth was at a depth not marked on the lead, the leads man called “By the deep six (or eight or eleven or whatever)”. When the depth fell between fathoms he would call “And a half five” for five and one half fathoms or “And a quarter less eight,” as called by Captain Hornblower’s leadsman in the story intro, signifies 7.75 fathoms.
And as we all know, the safe depth of a sidewheel riverboat on the Mississippi was two fathoms, which the leadsman would call out as “By the Mark Twain”.