9.

12.

15.
An acquaintance with this sort of LOGARITHMS--tables of which, calculated to a very high degree, are possessed by proprietors--will give us the key to the most puzzling problems, and cause us to experience a series of surprises.
By this LOGARITHMIC theory of the right of increase, a piece of property, together with its income, may be defined as A NUMBER WHOSE LOGARITHM IS EQUAL TO THE SUM OF ITS UNITS DIVIDED BY ONE HUNDRED, AND MULTIPLIED BY THE RATE OF INTEREST.

For instance; a house valued at one hundred thousand francs, and leased at five per cent., yields a revenue of five thousand francs, according to the formula 100,000 x 5 / 100 = five thousand.

Vice versa, a piece of land which yields, at two and a half per cent., a revenue of three thousand francs is worth one hundred and twenty thousand francs, according to this other formula; 3,000 x 100/ 2 1/2 = one hundred and twenty thousand.
In the first case, the ratio of the progression which marks the increase of interest is five; in the second, it is two and a half.
OBSERVATION .-- The forms of increase known as farm-rent, income, and interest are paid annually; rent is paid by the week, the month, or the year; profits and gains are paid at the time of exchange.