This is a one-off experimental type of zenith sector known as Airy's water telescope. It was designed by the Astronomer Royal, Sir George Biddell Airy and built by Troughton and Simms with the aim of testing the nature of aberration. The aberration of light is a phenomenon discovered by Astronomer Royal, James Bradley in 1725. It is the apparent shift in position of an astronomical object caused by the fact that the light from that object takes a finite amount of time to reach Earth while at the same time the Earth's position is constantly moving as it orbits the Sun. Aberration is generally treated as an error which, once identified can be accounted for in observations.

The telescope is a zenith sector designed so that the tube could be filled with water. It was used on the South Grounds of the Observatory between February 1871 and July 1873 and proved that the amount of sidereal aberration did not depend upon the medium through which light passed as had been previously thought. Specifically it was used to measure the coefficient of aberration when gamma Draconis was observed through approximately 36 inches of water.