The early chapters refer especially to the history of the steam engine, and to the theory of the action of steam in the cylinder of a steam engine, and the succeeding ones to the application of the theory in practice. Having felt personally the want of more practical informa tion on the subject than is contained in existing works, it has been the aim of the writer to supply such want, and to enable those who have not the opportunity of making experiments to gain a more intimate knowledge of the indicator. And it is hoped that the directions here given for the practical applica tion of this instrument will at the same time give the volume a considerable degree of interest to those engineers who are conversant with its ordinary working, but lack a knowledge of the principles involved.