It has been suggested many times that the secret to discovering how to make a perpetual motion machine will be found in Nature. One immediately thinks of gravity, but usually it is thought that that is not what those who expressed such an idea had in mind. They implied that if examples of perpetual motion did not exist in nature then it wasn't possible.
They declared that history shows that we (mankind) have found a way of obtaining usable motion from the force of the wind and of moving water etc. but for perpetual motion there had to be another force available to tap and since all forces were known and already utilised there could be no such device otherwise it would have already been invented.
It has been assumed that they sought some kind of mechanical action in nature. Examples such as the spinning of a sycamore seed as it falls to earth; the way grass and weeds could force their way up through concrete; the expansion of water as it freezes, breaking glass and pottery utensils; the rising tide lifting boats that were grounded at low tide; the expansion of steam in a boiler. All these actions and their causes were clearly evident and ways could be found to make use of them, but not gravity apparently.
Yet gravity is a force of nature. Without it the sycamore seed would not fall; the tides would not rise and fall. In fact none of the actions in nature mentioned above that have been observed have led directly to an invention. Of course there are examples of single-bladed propellers, not dissimilar to the sycamore seed, but that was not the first design derived from such an idea but was developed for other reasons later in the history of flight. I doubt that the windmill derived from observation of the effect of the wind on trees and falling leaves. It is more likely that initially the use of sails on sailing boats led to the use of windmill sails made of canvas, such as were and are used all over the middle East. Sails probably developed from seeing sheets of cotton or dyed materials, drying in the wind on lines, So although the action of forces on material things was observed and ways of using them discovered, finding ways of using gravity other than for falling weight clocks, not a continuous process such as was sought, was deemed impossible.
So what Bernouille and Boyle, for instance, were saying, was that we needed to find a force in nature to power the perpetual motion machine; not necessarily a specific mechanical action observed in nature. Of course perpetual is an ambiguous word; if a machine ran for ten years it would be as good as a perpetual motion machine; a windmill could be described as a perpetual motion machine as long as the wind continued to blow, but neither are perpetual in the literal sense of the word.
It has been assumed that looking for the answer in nature meant finding an example of perpetual motion in action rather than simply an available force such as gravity. Gravity is obviously that force and is really the only one left to us that might accommodate our aim - and it's entrance into the world of continuous propulsion is imminent.
In describing perpetual motion as being acceptable if it only lasted ten years, I omitted to point to the spinning of our planet earth as a perpetual motion machine, which has been spinning for a lot more than ten years; perpetual motion is all around us and is clearly continuous as far as we are concerned even if ultimately it stops.
JC
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