CHAPTER V $.$

DIFFERENCE ENGINE NO $.1.$

$"$ Oh nol we never mention it $,$ ?Its name is never heard,"

Vifference Engine No $.1-$ First Idea at Cambridge, $1812-$ Plan for Dividing Astronomical Instruments $-$ Idea of a Machine to calculate Tables by Differences $-$ Illustrations by Piles of Cannon $-$ balls.

Calculating Machines comprise various pieces of mechanism for assisting the human mind in executing the operations of arithmetic. Some few of these perform the whole operation without any mental attention when once the given numbers have been put into the machine.

Others require a moderate portion of mental attention: these latter are generally of much simpler construction than the former, and it may also be added, are less useful.

The simplest way of deciding to which of these two classes any calculating machine belongs is to ask its maker $-$ Whether, when the numbers on which it is to operate are placed in the instrument, it is capable of arriving at its result by the mere motion of a spring, a descending weight, or any other constant force? If the answer be in the affirmative, the machine is really automatic; if otherwise, it is not self $-$ acting.

Of the various machines I have had occasion to examine, many of those for Addition and Subtraction have been found