volume of the latter—i.e., the finer it is ground—the more slowly will the paint dry and the longer will it endure.

The correct and economic theory of metal painting is that the paint used for the priming coat should be of a preservative nature, i.e., of such a character that it will not only possess the power of inhibiting the corrosion of the metal but also that of absolutely excluding air and moisture therefrom, and that the other coats should be of a protective nature, i.e., that they will protect the priming coat from the deleterious action of rain, sunshine, and all other deteriorating agents.

In respect to the character of the pigments for the finishing coats, it is important that they be both chemically and physically inert, that they be ground very fine, and that they have an affinity for linseed oil. It is a matter of minor importance whether the pigment be graphite, lampblack, charcoal, oxide of iron, or what not, as far as durability is concerned. Where only three coats of paint are to be used, it is often advantageous to employ for the middle one a direct mixture of the priming and the finishing paints.

The character of the brush used is an important element in painting; for it is possible to ruin a coat of good paint by applying it with a broad, thin, flat brush. Instead, the paint should be well rubbed into the surface with a stout, full, round, bristle brush. "Less paint and more painting" should be the slogan for every painter who desires to do good work.

A temperature of about 70° F., combined with an atmosphere free from moisture, makes the ideal condition for both the applying and the drying of paint. The matter of humidity is of even greater importance than that of temperature; because nothing retards drying more than dampness and darkness.

For places that are badly ventilated, where sunshine does not reach, and that are damp or at times filled with steam or locomotive gases, it is economic to use a special kind of coating, preferably a varnish or resin paint so composed that it will dry rapidly.

This dissertation on paint and painting could be carried on almost indefinitely; but, if it were, the author might properly be open to criticism for departing from his subject of economics. It is possible that, even as it is, some of the readers of this chapter will accuse him of that fault. If so, he would reply that anything which treats of how to prolong the life of bridge paint or to protect the metal effectively against corrosion is directly in the line of true economics.

Summarizing in a very few words all that has been said herein upon the subject of "Economics of Metal Protection," it may be stated that it is always truly economic to use the best protective agencies procurable, irrespective of their cost, and to spend without hesitation all the money requisite for taking the very best of care of the metalwork in all first-class railway and highway bridges—or, for that matter, in any other bridges that are worth saving from speedy destruction.