Below the piston of the upper cylinder is an annular space E (surrounding the common piston rod) with a capacity equal to the maximum displacement of the liftram, while the corresponding annular area C of the piston of the lower cylinder is just large enough when subjected to the working water pressure to enable the work of lifting the net load to be done and any friction to be overcome.
Within this is hung loosely a second tube of much smaller dimensions so as to leave an annular space between the two.
To add to the fairly uniform distribution of stars between two planes a gigantic cluster of an annular or spiral form, also lying between the planes and completely surrounding the sun.
When the eclipse is total, there is a real geometrical shadow - very small compared with the penumbra (for the apparent diameters of the sun and moon are nearly equal, but their distances are as 370: I); when the eclipse is annular, the shadow is all penumbra.
To work the lift, pressure-water is admitted to the annular space C above the lower of the two balance pistons (the space B above the upper one is always in communication with the pressure-water), and the combined pressure on the two pistons is sufficient to lift the cage, ram and load.