Section XII. The vibrations must be similar

But if the vibration¹ be not similar at every impression, it can never be carried beyond the number of actual impressions; for, move any body as a pendulum², in one way, and it will continue to oscillate in an arch of the same circle, until the known causes make it rest; but if, after first putting it in motion in one direction, you push it into another, it can never reassume the first direction; because it can never move itself, and consequently it can have but the effect of that last motion; whereas, if in the same direction you act upon it several times, it will describe a greater arch, and move a longer time.

1 vibration   

n.颤动，振动；摆动

参考例句：

• There is so much vibration on a ship that one cannot write.船上的震动大得使人无法书写。
• The vibration of the window woke me up.窗子的震动把我惊醒了。

2 pendulum   

n.摆，钟摆

参考例句：

• The pendulum swung slowly to and fro.钟摆在慢慢地来回摆动。
• He accidentally found that the desk clock did not swing its pendulum.他无意中发现座钟不摇摆了。