Marine chronometer, No. 512.
|Following the work of John Arnold, the London watchmaker Thomas Earnshaw (1749-1829) further simplified the designs of the pocket and marine chronometers into their modern, readily reproducible form. This is a typical example of Earnshaw's marine chronometers, made from the early 1790s until his retirement in around 1820. The design is little different from the marine chronometer of the mid-20th century.|
|The sextant was first made in the late 1750s by the London instrument maker John Bird, then one of the finest makers in the city. He was commissioned to produce an instrument similar to the octant with a wider field of measurement. Like the octant, the sextant enables the angular distance between two points to be measured, for instance the Sun and the horizon or two stars. This example was made by one of the great instrument makers of the 18th century, Jesse Ramsden. The sextant is usually made of brass and has a very open frame design to reduce weight and wind resistance when in use.|
Ivory cross staff.
|The cross staff was first described in the 14th century and is thought to derive from a surveying instrument. It was taken up by navigators about a century later and is one of the earliest known Western navigational instruments. By moving one of the sighting vanes along the staff, angular measurements between the horizon and a celestial body can be read off the scale on the staff. Different lengths of vane are used according to the angle being observed. This relatively simple tool was the principle method of celestial navigation until the back staff came into use in the 17th century. They are usually made of wood allowing them to be made relatively quickly and cheaply. This rare example, however, made by Thomas Tuttell around 1700, is made from highly decorated ivory. It was probably a part of a gentleman's presentation set rather than for use on board ship.|
A back staff, c.1700
|The back staff was first described in 1595 by the navigator John Davis and became one of the principle instruments of navigation. It remained in common use until after the development of the octant in the 1730s. The measurement is taken by casting the shadow of the Sun onto the degree scale. It is designed to be used facing away from the Sun, saving the observer from having to look directly at it. They are usually made of wood allowing them to be made relatively quickly and cheaply. This rare example, however, made by Thomas Tuttell around 1700, is made from highly decorated ivory. It was probably a part of a gentleman's presentation set rather than for use on board ship. As well as being an instrument maker Tuttell was made Hydrographer to the King in 1700. This appointment was to prove fatal: he was drowned in January 1702 while making a survey of the River Thames.|
|The octant was developed in the early 1730s by John Hadley. The instrument enabled the observer to make more accurate measurements than had previously been possible with the backstaff. By using a reflected image the observer was able to measure two points simultaneously. The frame of this early example is made from mahogany and has a boxwood scale. Brass fittings hold the optics in position. There is no telescope and observations were made through a simple pin-hole sight. A small ivory plate in the middle of the instrument indicates that it was made by George Adams in 1753, at this time he worked from premises in Fleet Street at the corner of Racquet Court.|