We shall now enter into more detail respecting the principle illustrated by these rolling machines. The plasticity of the chocolate mass necessitates a rotation of the cylinder surfaces in opposite directions with dissimilar velocities. Accordingly two or more rollers are caused to work against each other, and in compliance with this principle of sub-division with differential velocities, their axles are fitted with wheels, of which each has a different number of cogs. So those rollers furnished with the greater number of teeth revolve more slowly, whilst in opposite instances there is a corresponding acceleration. [Illustration: Fig. 34.] The construction of the machines now in use differs more or less from that of the type first invented by Hermann, plan and elevation of which appear in figs. 33 a and 33 b respectively.[116] The granite rollers at _a¹_, _a²_, _a³_ are fitted with an octagonal iron axle that is somewhat thicker at the interior part and they are mounted upon a frame as shown in the drawing. The sockets of the central rollers _a²_ are fixed and each one is held in position by three sets screws; those of the two other rollers can be shifted along grooves in the frame and when the cylinders _a¹_ and _a³_ have been brought into proper position relatively to the cylinder _a²_ they are held fast by the set screws _p_. [Illustration: Fig. 35.] [Illustration: Fig. 36.] For the purpose of this adjustment, there is at each end of the machine a horizontal wrought iron shaft _f_ that can be turned by the winch _e_, and these shafts are fitted with two endless screws _d_ working in the corresponding wheels _c_. These occur on the spindles _a_, which screw in and out of the bearing blocks of the rollers _a¹_ and _a²_, but turn only in the fixed collars _b_ without being shifted from their place. The result is that on turning the cranks _e_ the corresponding cylinder _a¹_ or _a³_ is moved nearer to, or further from, the central cylinder _a²_, while the position of all of them always remains parallel. The shaft _Q_ is set in motion by the driving wheel _L_ fitted with the loose wheel _L¹_. It acts first upon the cog wheel _K_ which works in the larger wheel _J_ on the axle of the central roller _a²_. That works in the cog wheel _O_ and the wheel _P_ fitted to the roller _a¹_ driving them as well as the wheel _M_ and the pinion _N_ of the roller _a³_ The result is that the axle _a²_ makes 1¾ revolutions and _a³_ 6⅛ revolutions while _a¹_ in the same time makes only one revolution. [Illustration: Fig. 37.] The cacao or chocolate is supplied to this machine by the hopper _R_ which is placed between the rollers _a¹_ and _a²_. The pasty mass adhering to the rollers is carried forward by the quicker moving roller _a²_ and it is ground finer between the rollers _a²_ and _a³_, after which the material is removed from _a³_ at the outer side by an adjustable blade _gg_ and then falls down into a receptacle below.