This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
There are two kinds or classes of lewises, the straight-sided and curved. The latter is inferior to the former, as, when it is fixed or fitted into the stone, any sudden twitch or jerk of the supporting chain would tend to cause a fracture at point X (A, Fig. 208).
The hole for receiving the lewis should be cut so that a line down its centre should cross the centre of gravity of the stone. The splayed pieces of the straight-sided lewis (B, Fig. 208) are first fitted, and the centre piece last. A bolt as shown fixes their position, and also that of the hoop or ring by which the contrivance is to be elevated.
The sides or splays should fit accurately, otherwise they may flush the edge and break out (see C, Fig. 208). The risk of fracture may also occur if the sides do not fit.