The author has before him five photographic groups of machinists' societies and the large group of American and English mechanical engineers, as they were to be seen at one of their recent annual reunions.

A study of these scores and scores of faces suggests a thought or two:

1, but few of them seem to equal "the days of the years of the life" of the author;

2, each and every man has undoubtedly stored in the recesses of his brain one or many items of useful knowledge pertaining to the mechanic arts, unknown to the author of this work which, notwithstanding, is aimed to be educational;

3, that if every art of mechanism were for the time obliterated and known on earth no more, these men, modest as they are, could restore in a few brief years every useful art and manufacture;

4, throughout the groups appears a wise gravity born of the combined brain and muscle work going with the higher class of mechanicians.

It is to men represented by these photographic groups that the author appeals with profound respect for a kindly consideration of the contents of the work.

It is narrated of the good sculptor, Michael Angelo, that when at work, he wore over his forehead, fastened to his cap, a lighted candle, in order that no shadow of himself might fall on his work. It was a beautiful custom, and spoke a more eloquent lesson than he knew. For the shadows that fall on our work - how often they fall from ourselves.

So, it will he the aim of the editor and compiler of these succeeding pages to keep in the shaded background allusions to those long years of personal experience which have gone never to return but upon whose gathered and garnered experience the value of the work must rest.

The contents of the hook must, perforce, be its own justification; to be thorough and accurate is to be also honest, and to be all three, is worthy of the highest ambition, and such has been the endeavor of the author.

A book requires as much labor and careful thought as a complicated machine, and it often takes longer to produce it, and then, too, a reader wishes to know, first of all, what it contains, what ground it covers and what are its scope and limitations.