This is a four shaft adder as there are four main adding shafts. There are two shafts side by side that span the width of the adder at the bottom of the image. There are two more shafts between the right hand shaft of the two mentioned and the counter wheels. There are ten escapement wheels and associated solenoids per shaft giving a total of forty escapement wheels and solenoids. The number of teeth on each escapement wheel determines the value of the bet recorded. The more teeth the lower the value. The rotation of every escapement wheel is added by the epicyclic gear train connecting the escapement wheels such that the rotation of the adding shaft at any particular point along the gear train is the sum of the rotation of the escapement wheel at that point and every other escapement wheel upstream from that point, away from the counter display wheels.
Note that there is a paddle wheel at the end of a shaft on the left hand side of the adder just below the counter wheels at the back. This is an inertia brake. The counter wheels are arranged as a decade counter each counter wheel rotates one digit after ten digits have been displayed in the counter wheel on it's less significant digit side. When they were operating the units counter wheel could have such rotational velocity that it appeared as a blur. This is well demonstrated in the British Pathé Website video footage. As betting is erratic there was significant stress placed on the mechanical parts. The worst case scenario occurs when the race begins. The betting activity goes from its peak to nothing. Inertia prohibits machinery from stopping in an instant.This paddle, assisted braking by introducing aerodynamic drag, when the sell rate was rapidly declining. In addition, to control shock associated with inertia limited parts of the system there is a device called a storage screw, well covered in other images of this photo gallery. Briefly, the storage screw is a form of buffer memory which records transactions from the quick to accelerate adding shafts, and is read by the inertia limited parts of the adder which empty the buffer as they catch up. The horizontal tubular section on the right hand half of the adder immediately behind the first two adding shafts at the front, which span the width of the adder, looks like a short storage screw. This would buffer the transactions until the units counter wheel could catch up whether accelerating or braking.