30 DESIDERATUM IN MODERN PRINTING PRESSES SUPPLIED. 
Let us examine, tlierefore, what change 
takes place in this expression in conse- 
quence of a diminution of the angle ABA' 
by B A being depressed. We observe, then, 
that the denominator consists of two factors, 
of which the first (being the sum of the tan- 
gents of two angles, each of which is less 
than a right angle), decreases without limit, 
and becomes— O when B A coincides with 
B A'. The remaining factor, on the other 
hand, increases with the diminution of the 
angle ABA'; but its increase is limited by 
unity, which value it reaches when ABA' 
vanishes. Therefore, the value to which 
the expression approximates as ABA' de- 
creases, is 1, or ; and, consequently, the 
ratio ofP:W continually approximates to 
that of -g that is, the power applied 
at A, exerts at D, by the diminution of the an- 
gle A B A', a continually increasing force ; 
and this increase is without limit, for by 
sufficiently diminishing this angle, the force 
at D may be made greater than any that can 
be assigned. 
We have here, however, made no allow- 
ance for friction, and the imperfect rigidity 
of the materials employed ; and it is found in 
practice that these causes set bounds to the 
increased referred to long ere the attainment 
of such a force as that just mentioned. The 
latter of these causes, moreover, renders the 
exertion of some degree of power necessary 
to release the lever BA from its position 
after the pressure has been given. It is, 
therefore, found expedient not to reduce 
the angle ABA. below a value of from 
3*^ to 5*^ ; and to seek any further degree 
of force that may be necessary by other 
means. An examination of the formula in 
(4) shows that this is to be done either by 
increasing P or bv diminishing the ratio of 
BCtoBAandCD. 
It is considered a desideratum in all mo- 
dern printing-presses that the same power 
shall exert a continually increasing force 
and as we have seen that the principle de- 
monstrated above furnishes us with such a 
force, it is evident, that if vve can apply it pro- 
perly it is suitable for our purpose. I pro- 
ceed, therefore, to describe the press I have 
* The reason of this is, that in order to aiford 
room for the necessary motions and adjustments, 
the platten requires to move through a consider- 
able space. This condition cannot be ihlfilled by 
any ordinary combination of levers (or any other 
of the mechanical powers) without, at same time, 
making' it needful for the motive-power to pass 
through a space inconveniently great. Now, by 
such an arrangement as the above, in which the 
advantage gained is least when the platten meets 
with no resistance to its descent, and reaches its 
maximum only when the platten comes in 
contact with the face of the types, a greater 
space is on the Avhole described by the platten 
than would be the case were the force uniform 
throughout, and equal to that which is required 
to produce an impression. 
constructed, premising, that it is on a very 
small scale (just sufficient to print an octavo 
page), yet large enough to test the accura- 
cy of the principle, 
A,fig. 2, isa strong frame of wood, 21 
inches long, 9 broad, and 4 deep, to which 
are attached, by means of screws, and a bolt 
which goes completely through (he frame, 
and is fastented with a nut on the opposite 
side, the two cheeks B B. These cheeks 
are of wrought-iron, I| inches broad, and 
fths thick. They are connected at top by a 
strong iron bar, the ends of which are fixed 
into mortices in the cheeks. This bar is at 
the ends about the same breadth and thick- 
ness as the checks, but increases in thickness 
towards the middle for the purpose of al- 
lowing to be rivetted to it two pieces, C, 
between which, upon an axis passing through 
them, the lever D E moves. Another ob- 
ject served by the thickening of the connect- 
ing bar is to allow of the insertion of a screw 
Q, of which the part that projects under the 
bar, and with w'hich the prolongation of the 
lever D C comes in contact, by being length- 
ened or shortened has the effect of regulat- 
ing the pressure to be given, according to 
the nature of the matter. An iron plate is 
screwed to the under surface of the lever D 
E, to which the piece F, also of iron, I inch 
broad and fths thick, is rivetted. To the 
lower extremity of the piece F a bar G, 
fths of an inch broad and fth thick, is at- 
tached by a rule-joint. G is united at the 
other end by a similar joint to a plate H, 
which is screwed to the top of the platten 
The lengths of F and G, reckoning from the 
centres of motion, are and 3^ inches, 
respectively. The platten is of wood, 1| 
inch thick, having screwed to its under sur- 
face a plate of cast-iron, ground very flat ; 
and is preserved in its position, and confined 
to move parallel to itself, by guides K K, 
which pass through grooves in the cheeks, 
having just room enough to move freely 
without shake. These guides are of the 
form shown in fig. 3 ; and it is necessary 
to observe in regard to them, that the parts 
which pass through the cheeks must be so 
adjusted as to height, that a line drawn 
through the centre of motion at H, parrallel 
to the surface of the platten, §ha1l pass 
through a point in each, equally distant from 
the top and bottom. If this be neglected, 
these parts will be strained and liable to 
be twisted. L is a box, 10 inches long and 
8^ broad, which answers the purposes of 
chase and carriage. The sides are 1 inch 
thick ; such a degree of strength as is thus 
acquired being necessary to resist the pres- 
sure created in loching up the matter. Its 
depth is adjusted to type height ; and the 
bottom is about half an inch thick. It slides 
upon the frame A, being confined by a rim, 
about half an inch high, which goes round 
