Description
Instruments like this one may have had their scales engraved by Sarah Ramsden using her husband Jesse's innovative 'dividing engine'.

Sarah Ramsden is one of the best known 'craftswomen'. Her father was famous optician John Dollond. At 23, she married Dollond's young competitor Jesse Ramsden, forging a close tie between two of London's leading instrument makers.

But Sarah was more of a partner than a go-between. In 1775, Jean Hyacinthe de Magellan reported that Sarah had used Jesse's dividing engine to create precisely divided scales for sextants and other instruments.

Like Jesse, Sarah was skilled not only in this precise 'mathematical' work, but also in making optical instruments. In the 1780s, Jesse and Sarah separated. During this period, she returned to her own family, the Dollonds, assisting her brothers in their successful optical instrument making business.

Details of this unusual collaboration are scarce, but presumably Sarah Ramsden was able to teach her brothers techniques that she and Jesse had developed together.


Text based on the exhibition label by Joshua Nall for the exhibition "Craftswomen: Uncovering Hidden Labour in the History of Science" (2022-24).
06/11/2025
Created by: Hannah Price on 06/11/2025

Description
Today navigation instruments such as radar, radio and satellites update a ship’s position continuously. During the 17th and 18th centuries manual calculations had to be made using instruments such as the backstaff, octant or sextant.

The term “sextant” refers to an arc of 60°. The sextant is a portable instrument that measures angles between distant objects. It is an instrument that has been used in the fields of astronomy, surveying and navigation. The sextant was born as a compromise between the circle and the octant: it was an attempt to combine a larger arc with lightness and ease of handling.

When navigating, the sextant is used to measure latitude to an accuracy of 0.01 of a degree. To use the sextant the navigator moves the index arm until the index mirror appears to line up the sun within the horizon mirror. By reading the angle off the index arm, the angle of the sun (and therefore the ships latitude) can be calculated.

Much thought was put into the design for the sextant in an attempt to make them as accurate as possible. The first examples of sextants were made of brass and were mostly large and heavy. Over time the frame was designed to be rigid and light. A successful and popular design in the 18th Century was the “double-frame” or “pillar frame” sextant which was patented by Troughton in 1788. An example of this sextant can be seen in the navigation case.


The Search for Longitude
The sextant was also used in an attempt to determine longitude as well as latitude. In the 1750’s Tobias Mayer’s design of a reflecting circle was given to the British Board of Longitude who gave the instrument to Captain John Campbell to test fully at sea. Campbell liked the idea but found the circle too awkward to use. John Bird was ordered by Campbell to design a 60° arc (the sextant), which he thought to be adequate for the longitude measurements required. (To discover more about the search for longitude have a look at some of the books).