Make a magnetic power-split transmission, as used in the Owen Magnetic.
Both the generator and the motor armatures are connected to the output shaft, with the generator field connected to the crankshaft (and therefore spinning at engine speed).
Weaken the dynamo field, spin up the engine, and it’ll generate less voltage and more current, going straight to the motor to get you moving in “first”.
Weaken the motor field progressively, and it’ll generate less and less torque, and operate at higher speeds, while also pulling less power off the dynamo. Meanwhile, torque from the engine to the output shaft through the dynamo’s spinning will increase.
Short the dynamo brushes in “top” gear, and then you almost get total lockup (with an 80 RPM slip, apparently), with no power transmitted to the motor (and therefore no power splitting).
(You could actually take that further, hypothetically, and pull power off of the motor and put it into the generator - something that the Toyota hybrids actually do at very high speeds with very low engine power demand, to lower engine RPM. However, that would require more complex controls that likely weren’t seen as necessary in the 1910s.)
...also, while I’ve suggested another transmission - the Honda Juno scooter’s transmission - was the first power-split transmission in a production vehicle before, and was proven wrong by the original GM Hydra-Matic having a power-split torque pathway in some gears, this is decades before the original Hydra-Matic. (And, interestingly, I think it’s actually using a similar input slipping model to the Juno and later Honda HFT hydraulic power-split transmissions, to get the effects of planetary gearing without actually having planetary gears.)
Oh, and this transmission was brushed upon on here by RamblinRover Luxury-Yacht four years ago, but I thought I’d mention it because it’s quite neat, especially given the era.