For hundreds of years, sailors entering shallow water prevented their vessels from running aground by measuring the water's depth with a lead weight suspended from the end of a measured and marked line. Experienced leadsmen could produce quite accurate depth readings by gauging the vessel's speed, casting the lead forward, having it touch bottom, and then reading the markings as the line became vertical. This system was far from perfect, however, and John Guest's invention aimed to remove it inherent error. His own claims about the sounding guard's advantages are worth quoting at length.
"1st. It dispenses with the leadsmen, thus adding to the effective force for working ship and preventing the labor, hardship and exposure of men in the chains [where the leadsman stands] which in cold or stormy weather is sometimes unendurable.
"2nd. It is correct at any speed. It is well known to seamen that when a vessel is going rapidly over the bottom correct 'up and down' soundings are seldom procured. The ignorance, carelessness or mistakes of the leadsmen often produce the most disastrous results all of which are obviated by this 'sounding guard.' At night the difficulty in sounding with the lead and line is very great for the leadsman not being able to see the marks on the line can only feel and guess. This machine will act as well by night as day. In a seaway the lead and line give no correct soundings, while the machine going down with the ship always make[s] the least water.
"3rd. It is instantaneous in its action and constant. Thus the danger arising from the interval between one cast of the lead and the next is avoided. Ships often go on shore between the casts of the lead.
"4th. The depth of water under the bottom is known (when in soundings) without knowing the draft of the ship.
"5th. It is likely to produce a feeling of confidence and security in the navigator
which the lead and leadsman do not and thus enables him to devote his time and energies to working his vessel instead of the greater part being expended watching the leadsmen.
"6th. For surveying purposes in shoal water it is well adapted-saves time, labor and above all is accurate, presenting a constant register to the person recording soundings.
"7th In passing over banks or working up a channel at night, giving the alarm at the instant of approaching danger it is invaluable to the navigator.
Missouri native John Guest (1821-79) served as an officer in the U.S. Navy for most of his life and was a veteran of both the Mexican War and the Civil War. In 1856 he was a lieutenant temporarily stationed in Washington, D.C.
John Guest, Sounding-Guard for Vessels, U.S. patent no. 15,319, July 8, 1856.
"John Guest," John Howard Brown, ed., Lamb's Biographical Dictionary of the United States (Boston, 1900), 439-40.