Mr Watt m New Mkrometers. 
It is plain that this instrument possesses the advantage of 
measuring all distances with equal accuracy, until the imper- 
fection of vision at great distances interposes, as the Seale on 
which they are measured expands with the distances, and in un- 
even ground it possesses more accuracy than the chain, and is 
very valtiable 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 Tarbert, 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, to Mr 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 conse^ 
quence 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 or use of the method was posterior 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 ac- 
curacy. I made no further reclamation, as I perceived it was 
not in that court 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, I invented, I think, about 
that time ; it consisted of a thin prism, with its surfaces nearly pa- 
rallel, or inclined one degree or two, as in Plate III. Fig. 9. 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 at a, according to the dotted line, would re- 
fract that portion of the rays which passed through it, more 
than those which passed through the fixed part, and being pla- 
