Thomas Jefferson Papers

Enclosure: The Prompter, No. III: A Pendulum without a Bob, 3 January 1791

The Prompter, No. III
A Pendulum without a Bob

This must be a queer thing indeed. But the bob is of no use. The Secretary of State, in his report to Congress, on the subject of coins, weights and measures, has discarded the bob; and Mr. Jefferson’s mathematical abilities no man will dispute.

But what of a Pendulum without a bob? The Prompter answers, great effects indeed may be produced by it. This pendulum is to regulate the measures, weights and coin of the United States.

In a pendulum, with a spherical bob, it is difficult to ascertain the exact center of osullation; but in a cylindrical rod of uniform size, the center of osullation is just two thirds of the length from the point of suspension; (the very spot where the boys try to hit a ball with a hickory stick.) Now to apply the rod. Such a rod, vibrating seconds in the latitude of 45.° must be 58. inches and 7 tenths in length, nearly. Divide this rod into 587 equal parts, we have a line, ten lines make an inch, ten inches make a foot &c. This is an excellent method of measuring; decimal arithmetic being the easiest in nature. Besides, the rod will be a standard permanent and invariable, for a rod that will vibrate seconds must always be of the same length. Add to this, foreigners may learn our measures precisely by procuring a similar rod, and how gloriously convenient would it be for all nations to use the same measures.

But let us see how this bobless pendulum can regulate measures of capacity, weights and coins. A pendulum to vibrate seconds, must always be of the same length; divided into equal parts, those parts must always be of the same length. Then we have an inch—this is all we want—10 cubic inches may make a measure called a gill or a metre—10 metres a pint—10 pints a pottle or gallon—10 gallons a bushel or a cubic foot. How simple is this!

Then for weights: a cubic inch of clear rain water must, all the world over, be of equal weight—say then, a cubic inch of water is an ounce—10 ounces a pound—10 pounds a stone—10 stones a kental—10 kentals a hogshead.

Now for coins—let the dollar weigh an ounce (a cubic inch of rain water, which is nearly its present weight) which may be divided into ten parts for dimes—and these into tenths for cents, &c.

The Prompter is not jesting. It is a serious fact, that the Report of the Secretary of State, of which this is an epitome, is ingenious beyond any thing on the subject now extant. A pendulum without a bob, an invariable standard, may regulate the measures, weights and coins of all nations by one single principle.

Text from Connecticut Courant, 3 Jan. 1791.

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