ON BABBAGE’S ANALYTICAL ENGINE. 707 
Notre D.—Page 680. 
__ We have represented the solution of these two equations, with every 
detail, in a diagram* similar to those used in Note B.; but additional 
explanations are requisite, partly in order to make this more compli- 
eated case perfectly clear, and partly for the comprehension of certain 
indications and notations not used in the preceding diagrams. Those 
who may wish to understand Note G. completely, are recommended to 
_ pay particular attention to the contents of the present Note, or they 
_ will not otherwise comprehend the similar notation and indications 
_ when applied to a much more complicated case. 
In all calculations, the columns of Variables used may be divided 
into three classes :— 
Ist. Those on which the data are inscribed : 
Qndly. Those intended to receive the final results : 
$dly. Those intended to receive such intermediate and temporary 
_ combinations of the primitive data as are not to be permanently re- 
tained, but are merely needed for working with, in order to attain the 
ultimate results. Combinations of this kind might properly be called 
_ secondary data. ‘They are in fact so many successive stages towards 
_ the final result. The columns which receive them are rightly named 
_ Working- Variables, for their office is in its nature purely subsidiary to 
other purposes. They develope an intermediate and transient class of 
_ results, which unite the original data with the final results. 
____ The Result-Variables sometimes partake of the nature of Working- 
_ Variables. It frequently happens that a Variable destined to receive a 
- final result is the recipient of one or more intermediate values success- 
ively, in the course of the processes. Similarly, the Variables for data 
often become Working-Variables, or Result-Variables, or even both in 
succession. It so happens, however, that in the case of the present 
equations the three sets of offices remain throughout perfectly separate 
and independent. 
_ It will be observed, that in the squares below the Working-Variables 
_ nothing is inscribed. Any one of these Variables is in many cases 
~ destined to pass through various values successively during the per- 
_ formance of a calculation (although in these particular equations no 
instance of this occurs). Consequently no one fixed symbol, or com- 
- Dination of symbols, should be considered as properly belonging to 
7 
_ amerely Working-Variable ; and as a general rule their squares are left 
k. Of course in this, as in all other cases where we mention a general 
Tule, it is understood that many particular exceptions may be expedient. 
In order that all the indications contained in the diagram may be 
completely understood, we shall now explain two or three points, not 
hitherto touched on. When the value on any Variable is called into 
use, one of two consequences may be made to result. Either the value 
ay return to the Variable after it has been used, in which case it is 
eady for a second use if needed; or the Variable may be made zero. 
(We are of course not considering a third case, of not unfrequent oc- 
eurrence, in which the same Variable is destined to receive the result 
of the very operation which it has just supplied with a number.) Now 
e ordinary rule is, that the value returns to the Variable; unless 
it has been foreseen that no use for that value can recur, in which case 
a * See the diagram of page 711. 
