At the other extreme of the line is the selfstyled "practical man," who is so ignorant that he is unaware of his own short-comings, and who, after a certain age, becomes thoroughly case-hardened in his "views." He, of course, has abilities in his own peculiar line, but arrives at most of his conclusions by the art of "guessing." If he builds a machine he puts in sufficient metal to make two, making safe against breakdown to be sure, but never entertaining the slightest suspicion of proportion; or he may make some parts so weak that it is necessary to "brace up" the contrivance with numerous sticks of timber, to keep it together when the belts are on. Debating with the "practical man" is discouraging; his conclusions are all formed, and no possible discovery can modify them one whit; his mind is too inelastic to grasp new ideas; he meets reasoning with stubbornness, and however convincing an argument may be to a man who uses his wits, with the "practical man," whatever disagrees with his "creed," must be wrong somewhere. To sum up his weakness in few words, he is densely ignorant of first principles. Between these two extremes of character is the man who never grows too old or too wise to learn;" he sifts the information which is constantly coming to his senses, holds fast to that which is new or valuable, and promptly rejects that which is of no account. He is, of course, his own judge of quality in the operation, but experience and observation are his trusty allies. He studies the laws of nature, and settles in his mind as far as he can, the "first principles" of cause and effect with which his vocation brings him in contact, and when a difficult problem confronts him, he bases his efforts firmly on foundations that have been well tried and not found wanting; this mode of procedure is direct and to the point, and success follows as a matter of course. This book is intended for such men. A knowledge of the laws of ordinary natural forces and effects should form a part of the stock in trade of every mechanic; but a deplorable absence of such knowledge is constantly making itself manifest, and if these pages shall assist any seeker after "more light" to attain that end, the object of the writer will be fully accomplished. In conclusion the writer begs to return his thanks to his friends, Wm. H. Hoffman; T. M. Goodeve; Chas. A. Hague; "Chordal;" and also to "The American Machinist," and "The Engineer" (of New York), for many favors. Baltimore, Md., May 16, 1888. xlv Comparative Weight of Cork and Lead.. Table of Weight of Different Substances.. Table of Hardness of Substances. Improvement in the Economical Use of Fuel Mechanical Equivalent of Heat. Estimate of Heat given out in the Burning of Hydrogen and Carbon; Heating Power of a Substance.. Evaporating Power of One Pound of Ordinary Coal. Cubic Feet of Air Requisite for Each Pound of Fuel; Value of a Chimney in Promoting the Draught of a Fire. 64 Comparative Weight of Water and of Air... Considerations which Influence the Forms of Boilers.. 68 Estimate of the Breaking Strains of Riveted Joints of Forms of Joints Applicable to Tubes Subjected to Exter- nal Pressure, Illustrated and Described. 76 Standard of Comparison between different brands of |