This hand-operated sounding machine was designed for use while a ship was moving. The basic principle is the same as that of a lead and line, i.e. lowering a heavy weight until it hits the seabed. For this machine, the weight (usually a 24lb lead weight) was attached to wire wound onto a drum. The machine is fitted with winding handles and a brake.

The Kelvite Mark IV could be used with chemical tubes or by direct reading from the dial on top. In the first method, a glass tube lined with a chemical that changed colour on contact with water was tied just above the lead weight. As it was lowered, increasing pressure pushed water up inside the tube. Once the tube was brought back to the ship, the depth was read off against a wooden depth scale. Alternatively, a measure of depth could be obtained from the dial on top showing the amount of wire paid out, by using a table that converted this reading and the ship's speed into the true vertical depth. The dial is graduated to 300 fathoms.

It has always been essential for navigators to know the depth of water underneath their ship. The earliest method was to use a lead weight and line, but it was time-consuming and during the 19th century there were a number of attempts to mechanise the process. The first commercially successful device was patented by Edward Massey in 1802, and in 1876 Sir William Thomson (later Lord Kelvin) took out a patent (no. 3452) for his improved sounding machine. Between 1903 and 1906, Lord Kelvin worked with the Royal Navy to develop the Kelvite Mark IV Sounding Machine specifically for use on fast moving ships. This was adopted by the Royal Navy and was still being produced with only minor modifications in the 1960s. Both hand-driven and motor-driven versions of the machine were produced.