2.52 Mr. Babbage on a method of expressing 
drawings of the machine, in order that the two might mu- 
tually illustrate and explain each other. 
It is convenient in the three representations of a machine, 
to employ the same letters for each part ; in order to connect 
these with the notations, the letters which in the several 
drawings refer to the same parts, are placed upon the in- 
dicating lines immediately under the names of the things. 
If circumstances should prevent us from adhering to this 
rule, it would be desirable to mark those things represented 
in the plan by the ordinary letters of the alphabet, those 
pointed out for one of the other projections by the letters of 
an accented alphabet, and the parts delineated on the third 
projection by a doubly accented alphabet. In engines of so 
complicated a nature as to require sections at various parts 
as well as the three projections, this system is equally ap- 
plicable, and its advantage consists in this — that the number 
of accents on the letter indicates at once the number of 
the drawing on which it appears, and when it is intended to 
refer to several at the same time, the requisite letters may 
be employed and placed in the order in which the drawings 
will best illustrate the part under examination. 
The next circumstance which can be indicated by the 
system of mechanical notation which I propose, more readily 
than by drawings, is the number of teeth on each wheel or 
sector, or the number of pins or studs on any revolving 
barrel. A line immediately succeeding that which contains 
the references to the drawings is devoted to this purpose, 
and on each vertical line indicating any particular part of the 
machine, is written the number of teeth belonging to it. As 
there is generally a great variety of parts of machinery which 
