It is plain that this instrument possesses the ad vantage of measuring all distances with equal ac curacy, until the imperfection of vision at great distances interposes, as the scale on which they are measured expands with the distances, and in uneven ground it possesses more accuracy than the chain, and is very valuable in measuring distances from one hill to another, and across bays of the sea, where the chain cannot be used. which I experienced, in the survey of West Loch Tarbet, the northern shore of which is very much indented, and so rocky, that it is scarcely possible to measure a few chains in a straight line upon it.
I showed the instrument to all my friends at the time, and among others, Air. Smeaton, as I have mentioned; and used it publicly in my surveys, and it was consequently known to many people, though not published.
A Mr. Green, in 1778, applied to the Society of Arts for a premium for the same invention, which Mr. Smeaton apprized me of, and also informed the society of my claims, in consequence of which I was desired to attend their committee, where I informed them of what I had done, and at what time. Yet the society thought fit to award Mr. Green the premium, though his invention was pos terior to mine. I suppose because he had used an instrument of higher magnifying power, viz. forty, and consequently had been able to measure greater distances, and, as was said, with greater accuracy. I made no further reclamation, as I perceived it was not in that court that I could obtain justice; and as I dislike paper war, I did not apply to any other, though there were people then living who could have attested my use of it.
Another micrometer with a prism, 1 invented, I think, about that time; it consisted of a thin prism, with its surfaces nearly parallel, or inclined one degree or two. This prism was cut by a diamond into two parts, which, when they were fixed in the same plane, refracted all the rays which passed through them equally, but one of them remaining fixed, and the other moving on a centre, would re fract that portion of the rays which passed through it, more than those which passed through the fixed part, and being placed in the focus of the object glass of a telescope, two images were formed of each object, by a hich its diameter could be mea sured. An index, and divided sector of a circle, served to measure the comparative refractions.
This instrument I made with the sector and radius of wood, and gave it to Professor Anderson of Glasgow College; and, I suppose, it is still among his apparatus, which he left to a public institution.
The cross-haired micrometer, as described, leav ing me too much in the power of my assistants, where the distances were greater than permitted me to read off the number of chains on the rod myself, I thought of another about 1772 or 1773, which consisted of a telescope with an object-glass of a long focus, say three or four feet; this was placed in a tube with a slit in one side of it, nearly as long as the focus of the telescope, and the object glass being fitted to a short tube, which slid from end to end of the slit, could be moved backwards and forwards by means of a piece of metal fixed to the short tube, and coming out through the slit; a glass of six to nine inches focus was also fixed in the outer tube, of the nature of what is called a field glass, and to this was added an eye-glass, with a cross hair piece in its focus.
Now, it is evident, that if the object•glass be moved nearer the field-glass, their common focus will be shortened, and the image at the cross hairs diminished proportionally, until the glasses come into contact, when their common focus will be shorter than that of the field-glass alone; and two indexes fixed upon a rod being subtended by the cross hair. At any given distance, the same rod with its indexes being removed nearer the observer, upon sliding the object-glass nearer the eye, they may again be subtended by the cross hairs, and a scale on the side of the tube will show the com parative distance they have been removed, and the distance of the first object being known, that of the second will also he so. This scale could not, how ever, be a scale of equal parts, but one which could easily be laid down.
I made a rough model of this instrument at the time, but have never completed it, having been since engaged in concerns where such instruments were unnecessary. I described it, however, to several of my friends at the time, and among others, to the late Mr. Ramsden; but whether it has been thought of by any one else, or the problem pub lished, I am ignorant.* Another of Mr. Watt's minor inventions is his perspective machine, which he contrived in 1765, in consequence of his friend Dr. James Lind having brought from India a machine invented there by a Mr. Hurst. As this machine had many defects, Mr. 'Watt wished to make one more portable and easier in its use, and, at the suggestion of the late Dr. Robison, he turned his thoughts to the double parallel ruler, an instrument then very little known, and scarcely if at all used. After some reflection, Mr. Watt contrived a method of applying it for this purpose, and of making the machine extremely light and portable. A description with drawings of the machine, which Mr. Watt communicated to Dr. Brewster, was published by the latter in the Edinburgh Philosophical Journal.t About from fifty to eighty of these instruments were made at the time, and went to various parts of the world. Mr. George Adams, senior, copied some of those that went to London, and made them for sale.