. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. f a clock, as we look on its face, the apparentmotion at the south pole will be the opposite to. this. Again, if a pendulum is made to vibrate in the plane of ameridian at the equator, there will be no apparent change inthe plane of vibration, since it will always coincide with theplane of the meridian, and hence the pendulum will continueto swing north and south during the entire period of theeaiths rotation. The condition that the pendulum shall hereswing in the plane of a meridian is entirely unneces
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. f a clock, as we look on its face, the apparentmotion at the south pole will be the opposite to. this. Again, if a pendulum is made to vibrate in the plane of ameridian at the equator, there will be no apparent change inthe plane of vibration, since it will always coincide with theplane of the meridian, and hence the pendulum will continueto swing north and south during the entire period of theeaiths rotation. The condition that the pendulum shall hereswing in the plane of a meridian is entirely unnecessary, andis made only for the sake of illustration ; for there will be noapparent change in the plane of vibration, whatever may bethe direction in which the pendulum is made to vibrate. The apparent rotation, then, of the plane of vibration of thependulum is 360° in 24 hours at the poles, and notning at theequator. At places lying between the equator and the poles,the apparent angular motion of the plane of vibration will bebetween these two limits ; in other words, less than 360° in. 70 LINEAR VELOCITY OF ROTATION. 24 hours. Appropriate investigations show that the apparentangular motion of the plane of vibration at any place in anyinterval of time is equal to the angular amount of the earthsrotation in that time, multiplied by the sine of the latitude ofthe place* Thus, at Annapolis, we have for the angular motionin .one hour, 15° sin 38° 59 = 9° 26:so that the plane of vibration will make one apparent rotationat Annapolis in 38h. 09m. Such is the theory of the pendulum experiment. Now, numerous experiments have been made in different latitudes, and inevery case an apparent rotation of the plane of vibration fromeast to west has been observed, with a i&te agreeing very closelywith that demanded by the theory; and the conclusion is irre-sistible that the earth rotates on its polar axis, from west to ea<once in every sidereal day. 73. Linear Velocity of Rotation.—Tak
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