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We may now observe another advantage peculiar to this method of notation. It is, that all large numbers are divided into parts, in order to express them by the different orders of units, and then we add each different order separately, and without regard to its name, observing only that ten, in an inferior order, is equal to one in the next superior order. By this means we add thousands, millions, or any of the higher orders as easily as we add units. If on the contrary we had as many names and characters, as there are numbers which we have occasion to use, the addition of large numbers would become extremely laborious. The other operations are as much facilitated as Addition, by this method of notation.

In the above examples the numbers to be added have been written under each other. This is not absolutely necessary; we may add them standing in any other manner, if we are careful to add units to units, tens to tens, &c., but it is generally most convenient to write them under each other, and we shall be less liable to make mistakes.

In the above examples we commenced adding the numbers at the top of each line, but it is easy to see that it will make no difference whether we begin at the top or bottom, since the result will be the same in either case.

Proof. The only method of proving addition, which can properly be called a proof, is by subtraction. This will be explained in its proper place.

The best way to ascertain whether the operation has been correctly performed, is to do it over again. But if we add the numbers the second time in the same order as at first, if a mistake has been made, we are very liable to make the tame mistake again. To prevent this, it is better to add them in a reversed order, that is, if they were added downwards the first time, to add them upwards the second time, and vice versa.

*

The method of omitting the upper line the second time, and then adding it to the sum of the rest is liable to the same objection, as that of adding the numbers twice in the same order, for it is in fact the same thing. If this method were to be used, it would be much bet ter to omit the lower line instead of the upper one when they are added upward; and the upper line when added downward. This would change the order in which the numbers are put together.

The danger of making the same mistake is this: if in adding up a row of figures we should somewhere happen to say 26 and 7 are 35, if we add it over again in the same way, we are very liable to say so again. But in adding it in another order it would be a very singular coincidence if a mistake of exactly the same number were made.

From what has been said it appears, that the operation of addition may be reduced to the following

RULE. Write down the numbers in the most convenient manner, which is generally so that the units may stand under units, tens under tens, &c. First add together all the units, and if they do not exceed nine, write the result in the units' place; but if they amount to ten or more than ten, rescrve the ten or tens, and write down the excess above even tens, in the units' place. Then add the tens, and add with them the tens which were reserved from the preceding column; reserve the tens as before, and set down the excess, and so on, till all the columns are added.

III.

MULTIPLICATION.

Questions often occur in addition in which a

number is to be added to itself several times.

How much will 4 gallons of molasses come to at 34 cents a gallon?

34 cents

34 cents 34 cents 34 cents

This example may be performed very easily by the common method of addition. But it is easy to see that if it were required to find the price of 20, 30, or 100 gallons, the operation would become laborious Ans. 136 cents on account of the number of times the number 34 must be written down.

I find in adding the units that 4 taken 4 times amounts to 16, I write the 6 and reserve the ten; 3 taken 4 times amounts to 12, and 1 which I reserved makes 13, which I write down, and the whole number is 136 cents.

If I have learned that 4 times 4 are 16, and that 4 times 3 are 12, it is plain that I need not write the number 34 but once, and then I may say that 4 times 4 are 16, reserving the ten and writing the 6 units as in addition. Then again 4 times 3 (tens) are 12 (tcns) and 1 (ten which I reserved) are 13 (tens.)

Addition performed in this manner is called Multiplication. In this example 34 is the number to be multiplied or repeated, and 4 is the number by which it is to be multiplied; that is, it expresses the number of times 34 is to be taken.

The number to be multiplied is called the multiplicand, and the number which shows how many times the multiplicand is to be taken is called the multiplier. The answer or result is called the product. They are usually written in the following manner:

34 multiplicand

4 multiplier

136 product.

Having written them down, say 4 times 4 are 16, write the 6 and reserve the ten, then 4 times 3 are 12, and 1 (which was reserved) are 13.

In order to perform multiplication readily, it is necessary to retain in memory the sum of each of the nine digits repeated from one to nine times; that is, the products of each of the nine digits by themselves, and by each other. These are all that are absolutely necessary, but it is very convenient to remember the products of a much greater number. The annexed table, which is called the table of Pythagoras, contains the products of the first twenty numbers by the first ten.

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18

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88

96 104 112 120 128 136 144 152 160

27 36 45 54 63 72 81 90 99 108 117 126 135 144 153 162 171 180

10

20 30

40 50

60 70

80 90 100 110 120 130 140 150 160 170 180 190, 200

To form this table, write the numbers 1, 2, 3, 4, &c. as far as you wish the table to extend, in a line horizontally. This is the first or upper row. To form the second row, add these numbers to themselves, and write them in a row directly under the first. Thus 1 and 1 are 2; 2 and 2 are 4; 3 and 3 are 6; 4 and 4 are 8; &c. To form the third row, add the second row to the first, thus 2 and 1 are 3; 4 and 2 are 6; 6 and 3 are 9; 8 and 4 are 12; &c. This will evidently contain the first row three times. To form the fourth row, add the third to the first, and so on, till you have formed as many rows as you wish the table to contain.

When the formation of this table is well understood, the mode of using it may be easily conceived. If for instance the product of 7 by 5, that is, 5 times 7 were required, look for 7 in the upper row, then directly under it in the fifth row, you find 35, which is 7 repeated 5 times. In the same manner any other product may be found.

If you seek in the table of Pythagoras for the product of 5 by 7, or 7 times 5, look for 5 in the first row, and directly under it in the seventh row you will find 35, as before. It appears therefore that 5 times 7 is the same as 7 times 5. In the same manner 4 times 8 are 32, and 8 times 4 are 32; 3 times 9 are 27, and 9 times 3 are 27. In fact this will be found to be true with respect to all the numbers in the table. From this we should be led to suppose, that, whatever be the two numbers which are to be multiplied together, the product will be the same, whichsoever of them be made the multiplier.

The few products contained in the table of Pythagoras are not sufficient to warrant this conclusion. For analogical reasoning is not allowed in mathematics, except to discover the probability of the existence of facts. But the facts are not to be admitted as truths until they are demonstrated. I shall therefore give a demonstration of the above fact; which, besides proving the fact, will be a good illustration of the manner in which the product of two numbers is formed.

There is an orchard, in which there are 4 rows of trees, and there are 7 trees in each row.

If one tree be taken from each row, a row may be made consisting of four trees; then one more taken from each row will make, another row of four trees; and since there are seven trees in each

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