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This “Simple and Compound Interest Table, and Indicator for Any Amount From 1 Cent to 1000 Dollars at Six Percent From 1 Day to 7 Years” was prepared by L. Books, warranted accurate, and its copyright attested to by it being “Entered According to the Act of Congress by L. Brooks in the year 1849 in the Clerk’s Office of the District of New Hampshire.”
The device is no more than a simple read-out table of tabulated results of interest calculations. The principal for which the interest is to be calculated is set by the large figures set in sectors around the outer rim of the calculation disk. The Indicator (or long radial slot in the cardboard cursor) is placed over the desired amount and the interest is read off for different times specified along the cursor from 1 day nearest the centre, to 7 years at the outer periphery. The shorter slot is for interest on cents, giving the result in fractions of a cent.
Compound interest is given only for 2 to 7 years (above the blank space separator on the two slots). The remaining tabulation (below the blank space separator) is for simple interest.
A rule is provided for finding interest at another rate than 6 per cent. Namely: First, find the interest at 6 per cent by this table, and then, for 3 per cent deduct 1/2, for 4 per cent deduct 1/3, for 5 per cent deduct 1/6, and the remainder is the interest. For 7 per cent add 1/6, 8 per cent add 1/3, 9 per cent add 1/2, 10 per cent add 2/3, 11 per cent add 5/6, and for 12 per cent double the amount, and you have the interest.
The table is accompanied by an attestation D.H. Buffum, Cashier of Great Falls Bank, that he has examined the table and finds it agrees in every figure with a corresponding Book “and would cheerfully recommend it as better answering its design than any table for that purpose, for which I am acquainted.” The Book is presumably L. Brooks’ Average Tables (improved by J. W. Worthen) and both this instrument and the Book were published by J. W. Worthen & Co. of New York City, Boston Mass, and Concord New Hampshire.
On the reverse of the instrument is a Complete Time Table, tabulating the number of days between two dates in any year - presumably to be used in conjunction with the interest reckoner for determining interest accumulating between those dates.
Indicating the author’s confidence in the accuracy of his table, a $50 reward (a considerable amount for 1849 corresponding to about $1,500 in 20141) is offered by L. Brooks, “for the detection of an error in the table amounting to one cent”.
1 see http://www.davemanuel.com/inflation-calculator.php, viewed 12 October 2014 (↑)