Does a Plumb Line Always Hang Straight?
A vertical line at any point can generally be determined by suspending a plumb line, which is a weight attached to the end of a string, and allowing it to come to rest. The pull of the earth, or gravity, as we call it, will stop the swinging of the plumb line. If we could see the line of the pull of the earth we should see that it passes through the motionless bob, the string and the support from which the string hangs. As the string is in the same line as the earth’s pull, we say it hangs vertically.
At some places the plumb line does not hang quite vertically. Where this is so we know that it is caused by the action of some other force, such as the attraction of a great mass like a mountain, or by the gravitational pull of the moon, or by the centrifugal force of the earth’s rotation. Dr. Xcvii Maskelvne, a British astronomer, found that when he suspended two plumb lines near a mountain, one on the north and the other on the south, the angle between their directions was greater than the angle between two vertical plumb lines should be. On measuring he found that each plumb line was pulled slightly toward the mountain.
We can understand this; but in India a very strange thing happens. When a plumb line is suspended in the southern regions, it hangs quite vertically; but when taken north, it is pulled, not toward the Himalaya Mountains, but away from the massive mountains, toward the southern plain.
This behavior of the plumb line, so different from what we should expect, is due to the fact that the weight of the great table- land of southern India. and the material lying beneath it, is greater than the weight of the Himalaya Mountains and the material below them. The heavy plain attracts the bob of the plumb line away from the lighter mountains, massive though they appear to the eye. The unseen attraction is greater than the visible attraction.
