Vickers range clock The Vickers Range Clock was a clockwork device used by the Royal Navy for continuously calculating the range to an enemy ship . In 1903 , Percy Scott described a device he ' d invented which was similar to the Vickers clock . In April 1904 , Vickers worked with Scott and patented their device , samples of which were available for trials in 1905 . In 1906 , the Royal Navy ordered 246 units for use on their ships . More than one might be installed , to allow for tracking multiple targets . The device consisted of a circular dial with a single central rotating pointer , like a clock . The dial was engraved with the ranges in yards . The clockwork motor drove the pointer at a constant rate determined by a control on the right hand side of the device . This had a handle and its own dial , on which the rate of change of range ( or " range rate " ) could be selected . This was calculated by other means , often by use of a Dumaresq or a time - and - range plot . A second rotating handle was fitted to the left of the clock . This was connected by gears to the dial plate , which rather than being fixed as on a clock , could be rotated . One turn of the handle would rotate the scale relative to the pointer . Thus the clock could be set to the initial range to a ship , or corrections could be made to update the reading without disturbing the mechanism . The range on the dial was the range as passed to gunners for firing , and as such included any corrections necessitated by time of flight , wind , etc . , rather than being a true instantaneous record of distance to the enemy . By 1913 , a second red pointer had been added to the original black one , attached to it by a friction grip , which could be adjusted relative to the original hand , so that one indicated true range and the other the corrected range for targeting . Once some shots had been fired , spotters would report the fall of shell ( which could be observed because of the large waterspouts where the shells hit the sea ) and the range would be corrected up or down depending whether the shells were short or over the enemy . Initial estimates of range were obtained using optical rangefinders . The dial was fitted with three separate scales reading , and in 100 - yard steps , with marked subdivisions every . The handle for adjusting the dial was added from 1908 ; previously , the initial position of the dial plate had to be set by rotating it directly . Initial models had the rate setting dial calibrated in both knots and ' seconds per 50 yards ' . This was altered in 1909 to yards per minute , which standardised measurement was used on other equipment designed to work with the clock . An important aspect of the Vickers range clock , as well as other designs using a wheel - on - disc drive for the variable speed motor , such as the electrical clock used in the Dreyer Fire Control Table , was that it could only generate ranges according to a constant range rate . In most firing scenarios , the range rate would change continuously . In such cases , the operators of the clock would periodically alter the range rate in discrete increments as conditions suggested . Mechanism . Variable speed was obtained using a rotating metal disc driving a rubber wheel resting against it . The position of the wheel could be moved in and out along a radius of the disc , so that the wheel turned at different speeds ( faster the further out ) . Operating errors would occur from adjusting the wheel , as the wheel tended to slide as it was dragged across the face of the spinning disc as it was adjusted . The workaround adopted was to adjust the rate quickly and in a discrete manner , accepting that the generated ranges would deviate moderately from the desired continuous hyperbolic curves that predominated in use . In 1909 , alterations were made to the clockwork drive to improve reliability of the governed speed and to increase the power of the drive . A small aperture on the dial would reveal a visual tell - tale which indicate that the clock ' s spring required winding .