3. In this system the tubes are almost equally operative.

4. The extra body of water immediately over the furnace is both an

element of safety and a reservoir of power.

5. The outlet for the waste products of combustion is found in this

style of boiler in a more convenient position at _the rear end_ of the

boiler.

6. The boiler being self-contained, can be used in places where height

of story is limited.

[Illustration: Fig. 36.]