An interesting question from Museum Tom who discovered what looks like a position from a Depression Range Finder. Coastal Artillery question please, what is a Depression Range Finding Pedestal

Quite a simple trigonometrical principle, know the height of the range finder above sea level, measure the angle down to the target, apply a tangent formula and determine the range.

A bit more detail from Nigel aka Bombadier

"Some instrument which would discover the varying range of the target as it steamed through the water was absolutely necessary. The solution of this problem was entirely due to the inventive genius of Capt. H. S. Watkin R.A. who, whilst stationed at Gibraltar during the 'seventies (1870s) produced the Depression Range Finder and the Position Finder. Both of these instruments made their appearance during the 'eighties. The D.R.F., when laid on the bow-waterline of the target, recorded the range of the target as it moved through the sea, the P.F. both the range and bearing. The discovery and development of electricity enabled this vital information to be transmitted direct to the guns by the means of dials so that, in the case of the D.R.F. with the line-layer laying direct on his target, the gun was given the correct elevation, and in the case of the P.F. the correct line and elevation without the layer seeing the target at all. These instruments were gradually installed in the coast-defences at home and abroad during the 'nineties and the early years of the twentieth century, the P.F.s to serve the heavy guns (9.2") and the D.R.Fs the medium ones (6")."

From "The History of Coast Artillery in the British Army" by Col K.W. Maurice-Jones, D.S.O. late R.A.

Published by the Royal Artillery Institution 1959

Hope this adds something

Nigel

Engaging a moving target is very challenging - not only does the observer need to anticipate the movement, but slick drills are required in the calculation of the firing data in order to minimise the time taken between receiving fire orders and firing.

What is interesting in this case is that presumably the effect of varying levels of sea level must be taken into account.

This Depression Range Finder is at the Heugh Battery in Hartlepool,. The principle obviously worked, The Durham RGA hitting the Blucher during the bombardment of Hartlepool in December 1914.

Source: Coastal Artillery question please, what is