1822.] Mr. Babbage’s Letier to Sir H. Davy. 885 
presented by the engines would first interfere with their correct- 
ness. To remedy this evil, I have contrived means by which 
the machines themselves shall take from several boxes contain- 
ing type, the numbers which they calculate, and place them side 
by side; thus becoming at the same time a substitute for the 
‘compositor and the computer: by which means all error in 
copying, as well as in printing, is removed. 
‘«“There are, however, two sources of error which have not 
yet been guarded against. The ten boxes with which the 
engine is provided contain each about 3000 types ; any box hav- 
ing of course only those of one number init. Jt may happen 
that the person employed in filling these boxes shall accidentally 
pee a wrong type in some of them; as for instance, the num- 
er 2 in the boxes which ought only to contain 7s.. When these 
boxes are delivered to the superintendant of the engine, I have 
provided a simple and effectual means by which he shall in less 
than half an hour ascertain whether, among these 30,000 types, 
there be any individual misplaced or even inverted. The other 
cause of error to which I have alluded, arises from the type fall- 
ing out when the page has been set up: this I have rendered 
impossible by means of a similar kind.” 
The inventor of these wonderful contrivances next adverts to 
the quantity of typographical errors, in tables even of the great- 
est credit ; and then after remarking that to bring to perfection 
his various machinery would require a great expense, both of 
time and money, he describes the progress which he has made 
in these terms : 
“ Of the greater part of that which has been mentioned, I 
haye at present contented myself with sketches on paper, accom- 
panied by short memorandums, by which I might at any time 
more fully develop the contrivances ; and where any new prin- 
ciples are introduced [ have had models executed in order to 
examine their actions. For the purpose of demonstrating the 
practicability of these views, I have chosen the engine for differ- 
ences, and have constructed one of them which will produce any 
tables whose second differences are constant. Its size is the 
same as that which I should propose for any more extensive one 
of the same kind: the chief difference would be, that in one 
intended for use there would be a greater repetition of the same 
parts in order to adapt it to the calculation of a larger number 
of figures. Of the action of this engine, you have yourself had 
opportunities of judging, and [ will only at present mention a 
few trials which have since been made by some scientific gen- 
tlemen to whom it has been shown, in order to determine the 
rapidity with which it calculates. The computed table is pre- 
sented to the eye at two opposite sides of the machine; and a 
friend having undertaken to write down the numbers as they 
appeared, it proceeded to’ make a table from the formula 2? + 
w« +- 41. »In the earlier numbers my friend, in writing quickly, 
New Series, vou. 1v, 2c 
