Looking at the various postings I found myself wondering if there is a place on the forum for posting about little known items of ships equipment and how they were used.
As a starter, may I introduce the mechanical deep sea sounding machine. Even with the advent of the now ubiquitous "Echo Sounder" many ships right up into the early 1970s had these.
Whereas a "hand lead line" would normally be used to indicate a shelving sea floor ( or give an indication of the nature of the sea bed), the "deep sea" version could sound out depths well in excess of 100 fathoms....which made it ideal for letting the mariner know when a continental shelf was being approached. Before the days of (now defunct) Decca Navigator, Omega etc. when ships still relied on the sextant and chronometer, if the ship had been in overcast conditions for a few days then all navigation would have to be done by Dead Reckoning. Not always accurate. So any aid was useful. Even when Decca was around it didn't cover all coastlines.
The drawing is basically self explanatorybut 2 items may be puzzling.
1. The "arming cavity":- This is just a recess in the base of the 30lb lead sinker. This cavity would be filled with tallow or some such so that when the sinker hit bottom it would hold a sample of the sea bed. This would tell the navigator if the sea bed was of mud, shale, gravel or whatever. Compare that to the chart and you would have an idea (no more) of what area the ship was in.
2. As the ship would be moving at the time of casting, or if stopped an underwater current could carry the lead away; the length of wire paid out would or could be inaccurate. Hence the glass tube fitted into a brass cylinder. The glass tube was closed at the upper end and the inside was coated with a water reactive chemical. Pressure would force water up the tube to a particular point. On recovery the glass tube would be laid on the graduated "boxwood scale" and the length of the discolouration would be converted into fathoms.
Just a thought.