Mechanical Science. 223 
Thave also certain principles by which, if it should be desirable, a table 
of prime numbers might be made, extending from 0 to ten millions. 
Another machine, whose plans are much more advanced than several of 
those just named, is one for constructing tables which have uo order of dif- 
ferences constant. 
A vast variety of equations of finite differences may by its means be solv- 
ed, and a variety of tables, which could be produced in successive parts by 
the first machine I have mentioned, could be calculated by the latter one 
with a still less exertion of human thought, Another and very remarkable 
point in the structure cf this machine is, that it will calculate tables go- 
verned by laws which have not been hitherto shown to be explicitly deter- 
minable, or that it will solve equations for which analytical methods of so- 
lution have not yet been contrived. 
Supposing these engines executed, there would yet be wanting other 
means to ensure the accuracy of the printed tables to be produced by them. 
The errors of the persons employed to copy the figures presented by the 
engines would first interfere with their correctness. To remedy this evil, 
I have contrived means by which the machines themselves shall take from 
several boxes containing 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 con- 
tain each about three thousand types ; any box having of course only those 
of one number in it. It may happen that the person employed in filling 
these boxes shall accidentally place a wrong type in some of them; as for 
instance, the number two in tle boxes which ought only to contain sevens. 
When these boxes are delivered to the superintendent of the engine, I have 
provided a simple and effectual means by which he shall in less than half 
an hour ascertain, whether amongst these 30,000 types, there be any indi- 
vidual misplaced or even inverted. The other canse of error to which I 
have alluded, arises from the type falling out when the page has been set 
up ; this I have rendered imposs:ble by means of a similar kind. 
6. Strength of Cast Iron.—The increasing use of cast iron as 
a substitute for wood in building, has lately drawn considerable 
attention to the various circumstances affecting its strength and 
durability. Upon these subjects some interesting and import- 
ant facts will be found in Mr. Tredgold’s practical essay upon 
the above subject. We may daily observe among the new build- 
ings of the metropolis, entire houses which are stilted, as it 
were, upon iron columns, with a view of gaining space upon the 
ground-floor; and in many large buildings, the beams and roofs 
are entirely of iron, to the complete exclusion of all timber; it 
gives safety against fire, is not liable to sudden decay, nor soon 
destroyed by wear and tear; and it can be easily moulded into 
the form of greatest strength, or that best adapted for the in- 
tended purpose. It must, however, be remembered, that iron 
varies extremely in quality; that the method of casting mate- 
rially affects its strength; and that this is also greatly depend- 
ent upon many minute circumstances, which in ordinary cases 
are not attended to: such, for instance, as the exclusion of air- 
bubbles; the temperature of the moulds; and above all, the 
time allowed for cooling, which, when performed very slowly, 
