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91
If you wish to multiply [14v] one number by another, write the two
rows of characters in that way that the outermost figure of that number
which you multiply stands under the first character of the upper number,
and each of the others of the lower number are to the left. Next you shall
consider how much the larger figure which you want to multiply lacks
from ten. And as many units as are lacking from ten, that is how often
you should take the smaller number, that you want to multiply, from that
number of tens.72 So that you understand this, multiply seven and nine.
Nine is one less than ten, therefore you take one seven away from seventy,
and then sixty-three remains. That is seven times nine. In the same manner
you can attempt other numbers. Multiply the first figure correctly with
each figure which stands under and write above each figure the multiple
that it has, and to the left that which cannot be over it in the next place
using correct addition. Then when this figure is multiplied, move the
outermost of those that stand below under the next figure and multiply
with that as with the first. And, if multiplication makes it an article, set
a cipher over above and arrange the article to the left. But if both a digit
and an article result from the multiplication, then write the digit over that
figure which you multiplied and the article in the next place. But if only a
digit results from the multiplication, then write it over above. If a cipher is
in the upper number, then skip over it because nothing is a multiple of it.
And also take care of this, that you remove those figures as are put above
as soon as you have multiplied each, and write that digit in the place as be-
longs to each or a cipher if that is correct, and add that to that which stands
to the left side as remains. If a cipher stands over that figure which you
multiplied, then remove it if it becomes a digit after multiplication,73 oth-
erwise leave it standing in place. If you doubt whether you have multiplied
correctly, then divide apart the whole multiplied number with that number
which stood under. And you will get the same number as you had first.
If you want to divide apart some number, then write the two rows
of the characters and write the smaller number underneath. The larger
number must be two, three, or more times greater. Set the foremost figure
that stands under aligned with the first above and the others to the right,
continuing as long as the ones underneath last. Next think about how
72 The text does not mention for which cases this is a useful aid for multiplying digits.
73 The text has oc margfallda, as does Hauksbók, but af margfalldan would make more sense.
ALGORISMUS IN GKS 1812 4TO