+---------------------------------------------------+ | Steel rod and brass compensation, as 1: ·6091 | | Iron wire rod and lead compensation, ·4308 | | Steel rod and lead compensation, ·3993 | | Iron wire rod and zinc compensation, ·3973 | | Steel rod and zinc compensation, ·3682 | | Glass rod and lead compensation, ·3007 | | Glass rod and zinc compensation, ·2773 | | Deal rod and lead compensation, ·1427 | | Deal rod and zinc compensation, ·1313 | | Steel rod and mercury in a steel cylinder, ·0728 | | Steel rod and mercury in a glass cylinder, ·0703 | | Glass rod and mercury in a glass cylinder, ·0529 | +---------------------------------------------------+ It is evident that in this table the decimals express the length of a rod of the compensating material, the expansion of which is equal to that of a pendulum rod whose length is unity. As we are not aware of the existence of any work which contains instructions that might enable an artist or an amateur to make a compensation pendulum, we shall endeavour to give such detailed information as may free the subject from every difficulty. The pendulum of a clock is generally suspended by a spring, fixed to its upper extremity, and passing through a slit made in a piece which is called the cock of the pendulum. The point of suspension is, therefore, that part of the spring which meets the lower surface of the cock. Now the distance of the centre of oscillation of the pendulum from this point may be varied in two ways; the one by drawing up the spring through this slit, and the other by raising the bob of the pendulum. Either of these methods may be practised in the compensation pendulum, but the former is subject to objections from which the latter is exempt. Suppose it were required to compensate a pendulum of 39 inches in length, of steel, by means of the expansion of a brass rod. Here, referring to _fig. 204._, we have S C 39 inches (which is to remain constant) of steel; the pendulum spring, passing through the cock at S, is attached to another rod of steel, which is fixed to the cross piece R A at A. The other end of the cross piece at R is fastened to a brass rod, the lower extremity of which is fixed to the cock of the pendulum at B. Now the brass rod B R must expand upwards, as much as the steel rod A C expands downwards; and the length of the brass must be such as to effect this, leaving 39 inches of the steel rod below the cock of the pendulum. Let us first try 80 inches of steel. Multiplying this by ·6091, we have 48·73 inches for the length of brass, which compensates 80 inches of steel. But as 48·73 inches of the steel, equal in length to the brass, would in this case be above the cock of the pendulum, it would leave only 31·27 inches below it, instead of 39 inches.