Watt describes [1] the air pump he used in April 1776 as a jack head pump, shut at the bottom with a common clack bucket. By the force of its own weight, the bucket drops when the beam descends on the air pump's side. This is a significant design detail because the bucket hangs from the beam. A flexible suspension only permits lifting loads upward; it does not permit pushing loads downward. When the steam condenses in the main cylinder, the piston lowers the beam on that side, raising the beam on the air pump's side and raising the bucket along with it.

The air pump, which has a 20-inch (50.8 cm) stroke and a 7-inch (17.8 cm) diameter, produced a vacuum of 27 to 29 inches of mercury (0.036 to 0.038 atm). The displacement volume of the air pump was about 40 liters, assuming that the specified dimensions are the outer dimensions and a quarter-inch wall thickness. With each stroke, the main cylinder, which has a diameter of 18 inches (45.7 cm), raises 7.8 cubic feet (221 liters) of water. Sadly, Watt says nothing about the size or length of the main cylinder's stroke. Other times, it is asserted that the main cylinder volume of the steam engines was designed to pump half of its volume with each stroke. Therefore, the volume of the main cylinder is probably close to 440 liters. As a result, it can be deduced that the displacement volume of the air pump is roughly one-tenth of that of the main cylinder.


[1] Letter Watt to Smeaton, April 1776, cited in Farey, page 320