many devices for producing free circulation of the water in steam-boilers.
We can easily follow out the process of development. In Fig. 3 we see a simple U-tube depending from a vessel of water. Heat is applied to the left leg, and a steady circulation at once commences. In order to increase the heating surface we can extend the heated leg into a long incline (Fig. 4), beneath which three lamps instead of only one are placed. The direction of the circulation is the same, but its rate is increased.
A further improvement results from increasing the number of