Clitellum

Swollen part of the body of a leech near the perature. A common form was invented by Harrison, and from its shape is called the gridiron pendulum. The suspending part consists of an odd number of rods of two kinds of metal; e.g. , five, three iron and two bra . These rods are so arranged that the downward expansion of the iron bars is equal to the upward expansion of the bra . The lengths of the two sets must be inversely as their co-efficients of expansion, and thus the radius of oscillation will remain constant and the time of the pendulum will not be affected by any temperature changes. Another, invented by Graham, is called the mercury pendulum. The bob is a gla cylinder partly filled with mercury; the expansion of the rod downward is compensated by the upward expansion of the mercury. Mercury being more expansible than the material of the rod, the cylinder need not be of undue length. It may be effected by the differential expansion of a cro bar attached to the rod near the bob; a rise of temperature causes the bar to bend its arms up Clock-Pendulum, or Compensation Pendulum. season. ferior vertebrates. IMG:content-0910.png:[blocks in formation] of temperature produces the inverse effect.