EXPERIMENTS OX CENTEIFUGAL FORCE. 139 



from one pole to the other, that is, of the axis of 

 rotation, to be 7899*1 English miles, while the distance 

 from any point of the equator to the point exactly 

 opposite is 7925'G English miles, the former being thus 

 ;>G*5 miles shorter. This c compression of the earth,' 

 is due to centrifugal force, in consequence of the earth's 

 rotation. 



This effect of centrifugal force may be demonstrated 

 y means of a ring. R R, fig. 99 A, placed within the 

 frame of the whirling-table; a vertical spindle passes 

 through two opposite apertures in the ring, and is 

 firmly connected with it at the lower aperture, so that 

 ring and spindle rotate together; through the upper hole 

 the spindle passes loosely. If the apparatus be set to 

 rotate moderately, the ring presents the appearance of 

 a transparent globe ; this phenomenon is often observable 

 in rotating bodies of certain forms, and will be alluded 

 to, further on, in the chapter on Light. If the speed is 

 increased, the ring contracts in the direction of the 

 axis, and assumes an elongated shape, that of a com- 

 pressed globe, of which the outline is no longer a circle 

 but an ellipse. When the rotation ceases, the ring 

 returns to its circular form. 



The ring is made of a very thin sheet of brass, if possible only 

 ) mra 'l, or at most O mm '2 thick, of which a strip is cut, 44a long, and 

 ifrom 12 to 15 mm broad. Precisely at the middle of the strip, and 

 l cm from each end, a hole is punched, the edges of the holes are 

 aammered flat, and the ends soldered over one another so that the 

 ;wo end holes fit exactly over each other. A circular ring is thus 

 brmed with two holes through it opposite each other; these 

 aoles are widened with the rimer, until they jut allow the 

 ipmdle, a steel wire of about 3 or 4 mm diameter, to pass easily 

 trough. The soldered portion forms the highest point of the ring ; 

 it both sides of the lower aperture short tubes of brass are soldered 



