Gen . Roy’s Account of 
pedeftal, and alfo through a fmall fquare plate lying over the 
former, and fattened to it by three fcrews. In the under fide 
of this laft, there is a cavity for the projecting part of the 
tongue. This contrivance of the tongue with its dot was to 
guard againft any error that might arife from accidental mo- 
tion given to the inftrument between one obfervation and ano- 
ther, which from this precaution could never happen, without 
being immediately difcovered : for the wires being adjufted to 
their dots under the microfcopes refpeCtively, if the inftrument 
be then turned round i8o c , the wires will reciprocally bifect 
the dots that were originally oppofite to them, and thereby 
Ihew, that they are accurately in the diameter of the circle ; 
and fo on with regard to any other dots whatever. Hence 
this becomes the moft fevere mode of trying the juftnefs of the 
divifions of the inftrument. 
Art. XIV. Manner of reading of angles with the microfcopes . 
By attending to the magnified fcale at the bottom of the plate, 
it will appear, that the dot on the gold tongue, which is here in- 
verted, is about one minute to the left of zero, and alfo of the 
firft notch, with which the moveable wire alone co-incides. 
Now it will eafily be conceived, from what has been faid in this 
defcription, how readily, as well as accurately, any obferva- 
tion of an angle can be read off with fuch an inftrument ; for 
the degrees and quarters, that is to fay, the 30', or 45% 
being feen with the naked eye, and registered, the value of the 
fractional fpace between zero and the laft part grand divifion, 
feen in the field of the mierofcope, is obtained by turning the 
micrometer head until the moveable wire bifeCts the dot at 
that grand divifion. The number of notches towards the 
right hand paffed over on the fcale, equal to fo many revolu- 
tions 
