samples of the iron, from which the boiler is to
be built, tested for tensile strength, and take no
interested parties' word, or bond either, on that
point. An actual test settles the matter beyond
doubt. All holes should be drilled and rounded
in the inner edge, as shown at a, a, a, a, Fig. II,
as a drilled hole, if not rounded,
will shear the rivet sooner than a
punched hole. Flat surfaces should
be stayed to from six to ten times.
the pressure they are to resist. The
area of rivets holding crow feet to
boiler heads, etc., should be equal
to the stay coupled to said crow
foot. It is a good plan to make stays
coupled to crow feet slightly shorter
than the required length. Expand
them by heat, and, on cooling, they
will all be found tight and neces-
sarily bearing their proportional
strain. It is a bad plan to collect
one end of stays (such as crown
Fig. 12.
sheet stays when stayed to shell)
into a smaller area than the other, as it is
plain to be seen that the smaller area must re-
sist more than its share in proportion as its
area is smaller.

abc

CHAPTER IV.

BOILER ECONOMY.

THE Scientific measure of the economy of a boiler is the amount of water evaporated into saturated steam, at some unit of evaporation per pound of combustible. This unit, for convenience of comparison, is usually 965.6, or, the units of heat required for the vaporization of one pound of water, under a pressure of one atmosphere. However convenient this may be, it hardly ever expresses the amount of water which would be evaporated if the evaporation were to take place under one atmosphere, instead of being reduced to that condition from some other pressure at which it actually did occur. Neither is it a correct expression of the relative economy of different boilers, and particularly of different types of boilers, unless the evaporation takes place under the same pressure in each instance, and unless the pressure is the one at which the boiler is to be used. Manifestly a boiler designed to economically generate steam under a pressure of twenty atmospheres would not show equal relative economy when working under a pressure of two or three atmospheres, nor would the re

duction of the performance of two boilers, working under these pressures respectively to some standard unit, be in any sense a fair criterion by which to determine their relative economy.

The practical measure of the economy of a boiler in any event embraces more than this, it being necessary to take into account two, and sometimes all of the following considerations, viz., the amount of water evaporated into dry / steam of the required pressure, the amount of space occupied by the boiler and its appliances, the probable necessity of stopping for repairs, | 3 and the cost, in which is properly included the 14 expense of keeping in repair, renewing, etc. As an instance of the application of the foregoing, it would be useless to demonstrate that a boiler three times as large would increase the general economy of the locomotive engine, looked at in the light of the fuel consumed, because the economy in that case consists in getting a large amount of work from a very small boiler.

The space occupied and the steam generated take precedence, and must be first considered. On the other hand it would be an example of bad engineering, under ordinary circumstances, to place for permanent stationary purposes an ordinary locomotive boiler, and then by forced draught and unceasing attention get the same amount of work from it as in the case of the locomotive engine. It may thus be readily seen

that boiler economy under different conditions is the product of widely different factors. Theoretically, the subject has been widely speculated upon. Practically, however, it is always worth the consideration of the party who pays for the coal. One pound of good anthracite coal has a combustible efficiency of 15,000 heat units. Hence, if this could all be utilized it would evaporate under a gauge pressure of 75 pounds, 319.5°F and from a temperature of 60°, 13+ pounds of water. The average performance of boilers is hardly if any more than 1⁄2 of this. A consideration of what becomes of the unutilized heat is perhaps the best way to determine how to effect a saving. In the first place, it is not all wasted. Man cannot change the laws of nature. At the best he can only tack his little endeavors to these laws, and get them carried along as far as possible, and then submit to seeing them dropped. As in the cylinder of an engine the useful effect is limited to a difference of temperature expressed in a few degrees, so in the boiler, the useful effect must be obtained between the temperature of the fire, which may be termed the initial temperature, and that of the escaping gases, or the terminal temperature. Fortunately, these extremes-initial and terminal temperatures-may be, and usually are, much more widely separated in the case of the boiler than in the cylinder of the engine. Different temperatures in