The original Perpetuums... of which Drebbel may have built as many as 18, in different forms... drove a clockwork mechanism. They are not perpetual motion as we understand it today. They were powered by changes in air pressure and temperature, which caused a differential between the pressure of the air trapped in the sphere, and the atmosphere. As such, they were what is known as a 'thermoscope"... a bit thermometer, a bit barometer. But to a 17th century philosopher, the fact that these were driven by unseen forces, continuously, made them perpetual machines. They were promoted by Drebbel as being tapped into the driving force of the universe... the "Primium Mobile", whatever they conceived that to be. But Drebbel knew hydraulics, and the expansion of heated gases... he was of course quite aware of what drove his machines. But this was the time when magic held sway, and the proper presentation of your inventions, while holding back the reason they worked, kept you in great demand. Drebbel was favored by the patronage of two kings... and was famous for this, and other inventions. Not the least of these was the submarine. He also invented the thermostat, which was roughly based on the principles seen in this device.
Drebbel was also the inventor of the twin convex lens microscope, and he was a fine engraver, and much more.