4 



FoiicauWs Pendulum Experiment 201 



notwithstanding the revolution. Suppose the plane of vibration 

 to be the meridian of Greenwich : draw a series of lines across the 

 parallel of 30^, parallel to this meridian ; each of these lines will 

 show precisely the position of the pendulum, in the revolution, at 

 the point where the line intersects the parallel of latitude. If, for 

 example, we draw one of these parallel lines across the intersection 

 of 30^ with ihQJii'st meridian circle east of Greenwich; another 

 across the intersection of the parallel of 30^ with the second me- 

 ridian; another across the thirds and so on ; — each will mark the 

 position of the plane of vibration after a revolution of as many 

 degrees as are included between the meridians. Continuing this 

 on, the parallel lines become more and more easterly, and finally 

 they are exactly east and west in longitude 180^; or in other 

 words, the plane of vibration, which is still parallel to the Green- 

 wich meridian, is actually at right angles to the meridian of 180^; 

 this plane has therefore changed apparently 90^, while the earth 

 was revolving 180^ or making half its circuit. And if this be 

 continued, it will be found that the parallel lines will make the 

 whole circuit of 360^ in twice the earth's revolution, or what is 

 equivalent, in 48 hours. 



These parallel lines drawn on the globe, it is to be observed, 

 should be parts of great circles, and are strictly parallel to one 

 another only at the intersections with the parallel of latitude, like 

 the meridians where they cross the equator; their tangents at 

 these intersections will be actually parallel throughout, like the 

 tangents to the meridians at the equator, and these tangents rep- 

 resent the true angle of the plane of vibration. 



Again take latitude 70*^, and in the same manner draw lines 

 parallel to the meridian of Greenwich, going eastward. These 

 parallel lines will rapidly diverge from the meridian, the diver- 

 gence being nearly 9| degrees for every 10 degrees of longitude; 

 ^ Ion. 950 46^' E. the intersecting line will point east and west, 

 having made 90^ in its apparent revolution. In 191^ 33' it will 

 nave made 180^, and in 383° 6^ (or 23^ 6' east of the meridian 

 of Greenwich,) it will have completed the whole 360^; so that 

 in latitude 70^ the time of the apparent revolution of the plane 

 of vibration, is a little over 25J hours. The globe thus exhibits 

 to the eye the actual uniform position of the plane of vibration, 

 «*nd at the same time its apparent revolution. By marking these 

 parallel lines at one end with an arrow-head, and also describing 

 an arc with the point of intersection as a centre, the increasing 

 divergence from the meridians, and the apparent rotation by east, 

 south, and west to north, is well shown. 



If now tangents be supposed to be drawn to each meridian cir- 

 cle, at its intersection with any parallel of latitude, say that of 

 '0^, these tangents will intersect at a point in the axis of the 

 earth extended; and the angle included between any two such 



Second Series, VoL XII, No. 35.— Sept., 1861. 26 



