MACHINE FOR DRAWING IN PERSPECTIVE. 41 11 ide of a double parallel ruler, (HJK,) every member of which vas ten inches long between the centres, so that, when fully open, t formed two squares joined by one side of each, and in other tates formed lozenges, or rhombuses of different degrees of bliquity. This double parallelogram was formed partly of thin lips of wood, and partly of brass much hammer-hardened, and ll very light. To the middle (K) of the upper side of the higher parallelogram, was fixed, at right angles, upwards, another slip of wood, (L,) about eleven inches long, and ending in a brass boint which served for an index, which, by the construction, ould be moved equally easily in every direction, and with very ittle friction; and, at the same time, all the positions of the ulers were always parallel to each other. A pencil, pressed hpon by a spring, was fixed in the junction of the perpendicular slip or index at K. "A paper being stretched upon the board, and the sight beng moved to a proper distance from the board, (generally about eighteen inches,) the hand being applied to the pencil socket, for the pencil was not pressed upon by the hand,) the upper point of the index was led along the lines of the objects intended to be delineated; and, when perpendicular lines occurred, the index pointing to the upper end of them, the finger of the left hand being applied to the board touching the perpendicular slip, and the pencil drawn downwards, the line would be straight. In the same way, the horizontal slip. served as a guide for horizontal lines; all others were drawn by the eye guiding the index, and, if the paper was smooth, could be drawn very correctly. "The whole of the double parallelogram and its attached slips, (which latter were contrived to be easily separated from the board,) were made capable of being readily folded up, so as to occupy only a small space in the box formed by the board, when folded up. The sight-piece also folded up, and readily found its place in the box, which also contained screws for fixing on the legs of the instrument; and the box, when shut, could be put into a great-coat pocket. The three legs were made of tinned iron, tapering, and one a little smaller than another, so that they wer into one another, and formed a walking-stick about four and half feet long. "I made many of these instruments about the time men tioned, perhaps from fifty to eighty. They went to various parts of the world; among other places several went to London, where George Adams, senr., copied and made them for sale, putt ting his own name on them; and, as I have been told, in a boolk which he published, describing various instruments, he took the credit of the invention to himself, or expressed himself so as to leave that supposable; but I have not seen the book as far as I can remember, but have seen the instruments with his name or them." CHAPTER VI. R. WATT'S INTRODUCTION TO DR. BLACK, AND TO PROFESSOR ROBISON-DR. BLACK'S HISTORY OF THEIR FRIENDSHIP, AND OF MR. WATT'S IMPROVEMENTS ON THE STEAM-ENGINE-DR. ROBISON'S HISTORY OF HIS ACQUAINTANCE WITH MR. WATT— ACCOUNT OF HIS CHARACTER, ABILITIES, DISPOSITIONS, HABITS, AND. PURSUITS—— EXPERIMENTS ON MODEL OF NEWCOMEN'S ENGINE-INVENTION OF SEPARATE CONDENSER, AND FURTHER IMPROVEMENTS ON THE STEAM ENGINE. [T was at this time that the young artificer's earnest devotion to philosophical pursuits, as well as his amiable and virtuous disposition, gained him the approving notice and enduring friendship of Dr. Black, who, in 1756, was appointed Professor of Anatomy, and, in 1757, Professor of the Practice of Medicine, in Glasgow College. It was also in the commencement of the winter of 1758-9 that he made the acquaintance of another able and ardent student, imbued with predilections similar to his own, Mr. John Robison, afterwards, by Dr. Black's recommendation, appointed to succeed Dr. B. as Lecturer on Chemistry in the University of Glasgow; and who subsequently became eminent as Professor of Natural Philosophy in that of Edinburgh. Both of these learned persons, owing to the accidental circumstance of their testimony having been called for on occasion of infringements of Mr. Watt's patents, at a period nearly forty years subsequent to their first meeting at Glasgow, have left interesting narratives of the rise and progress of their intercourse with Mr. Watt, and of the origin of his first and greatest invention, which, as they do honour alike to their authors and their subject, we do not hesitate to place before our readers. That by Dr. Black, which is by much the shorter of the two, had never been noticed by any of the previous biographers of Watt, nor, indeed, did its existence appear to have been known to them; while from D Robison's longer, but highly curious and important narrativ only a very brief extract was published by M. Arago. History of Mr. Watt's Improvement of the Steam-Engine. "I became acquainted with Mr. James Watt in the year 175 or 1758, at which time I was Professor of Medicine and Lecture of Chemistry in the University of Glasgow. About that time M Watt came to settle in Glasgow as a maker of mathematical instru ments; but being molested by some of the corporations, who con sidered him as an intruder on their privileges, the University pro tected him by giving him a shop within their precincts, and by conferring on him the title of Mathematical Instrument Maker to the University. "I soon had occasion to employ him to make some thing which I needed for my experiments, and found him to be a young man possessing most uncommon talents for mechanical knowledge and practice, with an originality, readiness, and copiousness of invention, which often surprised and delighted me in our frequent conversations together. I also had many opportunities to know that he was as remarkable for the goodness of his heart, and the candour and simplicity of his mind, as for the acuteness of his genius and understanding. I therefore contracted with him an intimate friendship, which has continued and increased ever since that time. I mention these circumstances only to show how it happened that I was thoroughly acquainted with the progress of his inventions, and with the different objects that engaged his attention, while I remained at Glasgow, and, in a great measure, ever since. "A few years after he was settled at Glasgow he was employed by the Professor of Natural Philosophy to examine and rectify a * The original document is in the hand-writing of Dr. Black. On the enve. lope in which it is enclosed Dr. B. has written, "Mr. Watt's law-suit, 1796-97." DR. BLACK'S NARRATIVE. 45 mall workable model of a steam-engine, which was out of order. his turned a part of his thoughts and fertile invention to the ature and improvement of steam-engines, to the perfection of heir machinery, and to the different means by which their great onsumption of fuel might be diminished. He soon acquired such reputation for his knowledge on this subject, that he was employed þ plan and erect several engines in different places, while at the ame time he was frequently making new experiments to lessen he waste of heat from the external surface of the boiler, and from hat of the cylinder. "But after he had been thus employed a considerable time, he perceived that by far the greatest waste of heat proceeded from he waste of steam in filling the cylinder with steam. In filling he cylinder with steam, for every stroke of the common engine a great part of the steam is chilled and condensed by the coldness of the cylinder, before this last is heated enough to qualify it for being filled with elastic vapor or perfect steam; he perceived, therefore, that by preventing this waste of steam, an incomparably greater saving of heat and fuel would be attained than by any other contrivance. It was thus, in the beginning of the year 1765, that the fortunate thought occurred to him of condensing the steam by cold in a separate vessel or apparatus, between which and the cylinder a communication was to be opened for that purpose every time the steam was to be condensed; while the cylinder itself might be preserved perpetually hot, no cold water or air being ever admitted into its cavity. "This capital improvement flashed on his mind at once, and filled him with rapture; and he immediately made a hasty trial of it, which satisfied him of its value, employing for this purpose a large brass syringe which he borrowed from a friend.” Such is the first part of the concise, but emphatic and comprehensive account given by Dr. Black; the remainder of which we reserve till somewhat later in our narrative. In the meantime, we proceed to give the greater portion of that of Dr. Robison, which, entering more into detail, seems more entirely to place us, |