TO THE MICROSCOPE. 131 



pull the weight aside in a direction PO perpendicular to the 

 plane of the strings, so that this time all three strings swing about 

 the line AB (Fig. 4). The weight will again begin to swing 

 like a pendulum, but in a direction 00 ' perpendicular to its 

 former direction, the chief difference being that it does not 

 swing quite so rapidly as before — -each oscillation takes a little 

 longer. This difference in the rate of oscillation may be intensi- 

 fied — and so better exhibited — by making the upper strings {AC^ 

 BC) of considerable length and the lower string {CF) short. 

 But for what follows it will be necessary to do the reverse, by 

 arranging the strings so that the knot C is not more than an inch 

 below the supporting-bar AB, while the lower string CF is of 

 considerable length. 



Now let the weight be pulled aside in a diagonal direction, say, 

 at an angle of 45*^, with the plane of the three strings. Let this 

 direction be represented in Fig. 5 {a) by the diagonal line //', 

 where 00\ EE\ represent the directions in which the weight 

 oscillated in the former cases. On letting the weight go this time 

 it will begin by oscillating along the diagonal //', but it will not 

 continue to do so for any length of time. On the contrary, the 

 motion will go through the series of changes represented in Figs. 

 5 {a-k). After a little while the weight will begin to swing a little 

 from side to side of the diagonal II', and will then revolve in an 

 oval curve or elHpse as at {b). Gradually this ellipse will widen 

 out until the weight revolves in a circle (c). After this the path 

 of the weight will elongate along the diagonal UU^, and contract 

 along II' {d), until a time comes when the weight swings to and 

 fro along the opposite diagonal, UU', as at (<?). Subsequently, at 

 (/) it will begin to revolve in an ellipse, but in the opposite direc- 

 tion to what it did previously. This ellipse will contract along the 

 diameter UU', and will open out along the diameter 11'^ and will 

 again pass through the form of a circle {g). The path will then 

 elongate as at (/z), until at {k) the weight once more swings to and 

 fro along II', just as it did at starting. After this the same cycle 

 of changes will be repeated. As long as the weight keeps swing- 

 ing, the path in which it moves will keep changing periodically 

 backwards and forwards from one diagonal to the opposite one, 

 each time passing through all the intermediate forms of ellipses 

 and circles shown in Fig. 5. 



