cycles and even continuous operation can be achieved by using the preferred embodiments shown in fig2 a 2b and 2c in the fig2 a embodiment the transparent tube 220 is well insulated for example it is made of thick low thermal conductivity plastic such as polycarbonate or a double walled glass tube the objective is to keep heat within the tube so that liquid and the air above the liquid will stay close in temperature if the liquid heats too much above the air temperature it may heat the air in the diver 201 to the point where the diver even shielded from the heat source is too buoyant to descend a second advantageous feature of the fig2 a embodiment is that no more than about 60 of the tube volume is filled with liquid 210 the advantage of this partial filling is that it prevents large fluctuations in the pressure of the air above the liquid as the liquid heats it expands confining the air above to a reduced volume this in turn increases pressure in the tube against the air in the diver if the tube is nearly filled with the liquid these pressure increases can dominate the thermally induced pressure increases in the diver and prevent the diver from rising in the fig2 b embodiment the tube 220 has an enlarged.