We should make the length of tubes never more than fifty diameters, and forty-eight diameters is a good standard for length.

As far as safety is concerned, the plain cylindrical fire tube boiler is not surpassed, as the records of boiler explosions will show upon investigation.

There are many of this style of boilers that have been in constant use for over twenty years, and they are safer to-day than several other kinds entirely new.

Figs. 34, 35, 36 show a working drawing for a 200 H. P. boiler from a design by Wm. H. Hoffman, M. E., Passaic, N. J.

The tube opening is sub-divided fifty-four times. The boiler has no steam dome, a liberal sized dry pipe being substituted for dome.

In the setting it will be noticed that there are two side walls on each side of the boiler, with an air space of 3" between them. The bridge wall is hollow and wide, the space admitting a sufficient quantity of air to the combustion chamber, just behind the bridge wall, to ignite any of the escaping gases.

The heat after leaving the front end of the tubes passes over the top of boiler shell to main chimney flue, there being a thin layer of asbestos on the shell to protect it from any undue rise in temperature. It will be noticed that the brick work does not touch the boiler at any point, they being separated by thin cast iron plates.

The buck-staves are made of wrought channel iron instead of cast iron, with the flat face toward the brick work.

The double wall system will prevent cracking, and is a great source of economy, as the air space becomes a perfect non-conductor.

Two doors are introduced in the fronts for convenience of attending to fires, but there is no partition in the furnace. The boiler can, by the peculiar method of arranging tubes, be thoroughly cleaned inside as well as outside.

Fig. 37 shows the Babcock and Wilcox water tube sectional boiler, which has a world wide re

putation as the best boiler of that type yet brought out. For a full description write to Messrs Babcock and Wilcox, (New York City) for a copy of their valuable little book called Steam." They will send you a copy free of

Babcock and Wilcox's Sectional Boiler.

This boiler consists of a series of inclined wrought iron tubes connected by T heads, which form the vertical water-channels at each end. The joints are faced by milling them, and then ground so perfectly tight that a pressure of 500 pounds to the square inch is insufficient to produce leakage. No packing is used. The fire is made under the front and higher end of the tubes, and the products of combustion pass

up between the tubes into a combustion-chamber under the steam and water drum; hence they pass down between the tubes, then once more up through the space between the tubes, and off to the chimney. The steam is taken out at the top of the steam-drum, near the back end of the boiler. The rapid circulation prevents to some extent the formation of deposits or incrustations upon the heating-surfaces, sweeping them away and depositing them in the mud-drum, whence they are blown out. Rapid circulation of water, as has been shown by Prof. Trowbridge, also assists in the extraction of the heat from the gases, by the presentation of fresh water continually, as well as by the prevention of incrustation.