160 TEE POPULAR SCIENCE MONTELY. 



similar to those of the external air, whose average temperature for 

 each interval is given in the table. 



The compensating action of the pendulum evidently depends upon 

 the relative lengths of steel and zinc, and it is easily possible that 

 great difficulty would be experienced in cutting and fitting tubes of 

 exactly the right length ; to complete the adjustment a very delicate 

 contrivance is added. 



Two compound bars of brass and steel (h and i, Fig. 1), with small 

 weights at their ends, are hung to the crutch-axis by means of a collar 

 loose enough to be easily turned. The rods are so made that under 

 normal conditions the brass and steel are of the same length, and the 

 two bars are in the same straight line ; the center of gravity of the 

 rods and the weights (regarded as one body) is therefore in the axis, 

 and the weights are balanced in every position, no matter what angle 

 the line of the rods makes with the plane of the horizon ; they affect 

 the pendulum only by their inertia. But, when a change in tempera- 

 ture occurs, the brass and steel become of unequal length, owing to a 

 difference in the co-efficients of expansion of the two metals, the rods 

 are bent, and the center of gravity of the rods and weights is no long- 

 er in the axis, nor is it in the same vertical plane as the axis except 

 when the weights are in a horizontal line ; so that an unbalanced force 

 is introduced whose compensating action varies from a maximum 

 when the weights are in a horizontal line, to zero when the weights "are 

 in a vertical line. To be explicit, suppose the rods to be horizontal 

 and the brass uppermost, and let there be an increase of temperature. 

 The brass will expand more than the steel, and, the rods being bent 

 downward, the weights will be lowered. As the pendulum swings 

 the weights swing with it, and are continually trying to get back to a 

 horizontal position where they would balance each other ; if they were 

 swinging alone, they would evidently swing faster than the pendulum, 

 and therefore, being attached, they accelerate its motion. If the steel 

 were uppermost, the weights would be raised with an increase of tem- 

 perature and the pendulum retarded. If the rods were both vertical, 

 a change of temperature would only throw the center of gravity of 

 the two weights to one side or the other of the axis, but would not 

 raise or lower it ; this would only introduce a continuous force tend- 

 ing to make the pendulum oscillate farther on one side than the other, 

 but not affecting its rate. At intermediate positions between the ver- 

 tical and horizontal, the change in the position of the center of gravity 

 due to a change of temperature would vary with the angle made by 

 the line joining the centers of gravity of the two weights with the 

 plane of the horizon ; any required compensating action, between the 

 limits above mentioned, for a known change of temperature, can there- 

 fore be obtained by setting the rods at the proper angle. 



In order to make a small change in the rate without stopping the 

 pendulum, the device shown in Fig. 1 has been employed : A weight 



