# Page:Scientific Memoirs, Vol. 3 (1843).djvu/697

687
L. F. MENABREA ON BABBAGE'S ANALYTICAL ENGINE.

a single one $\scriptstyle{(\text{A}b+\text{B}a)x^{m+q}}$. For this purpose, the cards may order $\scriptstyle{m+q}$ and $\scriptstyle{n+p}$ to be transferred into the mill, and there subtracted one from the other; if the remainder is nothing, as would be the case on the present hypothesis, the mill will order other cards to bring to it the coefficients $\scriptstyle{\text{A}b}$ and $\scriptstyle{\text{B}a}$, that it may add them together and give them in this state as a coefficient for the single term $\scriptstyle{x^{n+p}=x^{m+q.}}$

This example illustrates how the cards are able to reproduce all the operations which intellect performs in order to attain a determinate result, if these operations are themselves capable of being precisely defined.

Let us now examine the following expression:—

$\scriptstyle{2.\frac{2^2.4^2.6^2.8^2.10^2.\ldots.(2n)^2}{1^2.3^2.5^2.7^2.9^2.\ldots.(2n-1)^2.(2n+1)^2}}$,

which we know becomes equal to the ratio of the circumference to the diameter, when $\scriptstyle{n}$ is infinite. We may require the machine not only to perform the calculaiton of this fractional expression, but further to give indication as soon as the value becomes identical with that of the ratio of the circumference to the diameter when $\scriptstyle{n}$ is infinite, a case in which the computation would be impossible. Observe that we should thus require of the machine to interpret a result not of itself evident, and that this is not amongst its attributes, since it is no thinking being. Nevertheless, when the $\scriptstyle{\cos}$ of $\scriptstyle{n=\infty}$ has been foreseen, a card may immediately order the substitution of the value of $\scriptstyle{\pi}$ ($\scriptstyle{\pi}$ being the ratio of the circumference to the diameter), without going through the series of calculations indicated. This would merely require that the machine contain a special card, whose office it should be to place the number $\scriptstyle{\pi}$ in a direct and independent manner on the column indicated to it. And here we should introduce the mention of a third species of cards, which may be called cards of numbers. There are certain numbers, such as those expressing the ratio of the circumference to the diameter, the Numbers of Bernoulli, &c., which frequently present themselves in calculations. To avoid the necessity for computing them every time they have to be used, certain cards may be combined specially in order to give these numbers ready made into the mill, whence they afterwards go and place themselves on those columns of the store that are destined for them. Through this means the machine will be susceptible of those simplifica-

VOL. II. PART XII.
2 z