The model presented here is of the kind designed by John Harrison; it has an overall length of 48.5 cm and is composed of a brass lens (diameter 12 cm) held by an iron rod by means of two frames: one consists of two iron rods and the other of two brass rods; they are closed by cross pieces, they too made of brass. The two frames have in common the lower cross piece and the one made of bronze is contained in the iron one.
At the centre of the upper crosspiece of the iron frame there is a suspending hook and the lens is suspended from the centre of the brass frame. The iron rod from which the lens is suspended travels freely at the lower crosspiece common to the two frames, while the upper crosspiece closing the brass frame travels along the iron rods; thus, when the temperature increases, the iron rods can increase in length only in the downward direction, while the brass ones can increase in length only upwards.
If we take into consideration the law of linear dilation we can immediately see that it is sufficient, at zero degrees, for the ratio of the total length of the iron rods and that of the brass rods to be equal to the inverse of the ratio between the dilation coefficients for their dilation to be equal at any temperature.
In such a way the lengthening of the brass rods raises the lens by an amount equal to the length by which the iron rods lower it; consequently, the length of the pendulum remains constant and the constancy of the period of oscillation, even for large variations in temperature, is assured.
Daguin (1878), T. II, p.198
Drion - Fernet (1883), p.209
Ganot (1864), p. 217
Privat Deschanel (1869), p.217
Turner (1987), pp. 140, 285