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LETTER XI. 
IN my laft letter, I endeavoured to give 
you a clear idea of the feveral contriv- 
ances for the addition, or accumulation, of 
numbers or quantities. They are all found- 
ed on the fame principle ; their object is the 
fame, and, being thus linked together, mu- 
tually illuftrating each other, feem moft 
likely to imprefs the mind fcientifically. I 
fhall now proceed to SUBTRACTION, the 
rationale of which you will eafily compre- 
hend. The defign of this operation is, by 
taking a lefs number from a greater, to dif- 
Cover the difference. For example, 
From 769 
Take 436 
Sum 333 
Beginning with the firft figure on the 
right, I fay 6 from 9 there remains 3 ; then 
3 from 6, there remains 33 and laftly, 4 
from 7 and the remainder is 3. To prove 
that I am right, I add thefum to the lefs of 
the two numbers above, and find, that to- 
gether, they make 769. 
Again: 
