...that is a natural one can be an usefull suggestion for our search to collect gravity power.Ocean mechanical energy of the tides are driven primarily by the variable gravitational pull of the Moon.His gravity field come and go(due to the modifiable turning radius around the Earth) and causes the tides to rise and fall as it travels in its orbit.This way ,the tidal energy works from the power of changing tides,in particular,this up-down flows are assignable to a variable potential field.In our constant gravity ,we can pull out a field fluctuating "effect"(like that of the Moon) "via" arm torque(in reality,that's all that we need...).Only one problem:the loop,the self motion.Briefly,this natural design has a pulsatory gravitational pull(driver) and tides waves(follower).To harvest the "free" power of the  gravity,let's imagine a driver,something as the natural tides,waves "due to"  the Earth's field...the store-restore energy system(oscillatory "column") can be ,let's say a pulsatory Watt's governor.Remind:the driver must act as a waver maker(tides simulator).Somebody(Minsky)  said:...and if the Cosmos is as  a huge "Computer"(Laws "Maker")?Smile:we need "machine language translators"(remind Deus ex Machina)?.That's all ,above us ...All the Bests!/Alex