^62 Des eviction of a Perspective Instrument . 
be again unfolded, and another point of the object is to 
be ascertained in the same manner as before ; the space 
between these points may he then connected with a line ; 
fresh points should then be taken, marked with a pencil, 
and connected with a line ; and so on successively till the 
whole object is delineated.” 
The above machine affords a delineation which is 
strictly accurate : but I take this opportunity of mention- 
ing one still more portable, though less exact, which may 
be used in taking small sketches in the field ; where the 
table, and fixed sheet of paper cannot always be supposed 
at hand. I do not know the contriver. It is merely a 
strait flat ruler, having a division of inches and small 
parts (or any other division) on its edge. A string is fas- 
tened to the middle of the ruler by passing it through a 
hole, and tying a knot on the other side ; and at the other 
end of the string there is a small bead or knot to be held 
in the mouth. The length of the string may be adjusted 
at pleasure ; and when the ruler is used, it is held up at 
right angles to the stretched string, so that its edge, as 
seen by one eye, may apply to any two objects; between 
which it will show the distance to be afterwards transfer- 
red upon the paper by a scale, or by estimate. 
In this use of a graduated rule, it is most convenient 
and accurate to select some one object in the picture for 
the point of sight, and to measure all the distances from 
thence from the middle or beginning of the divisions 
where the direction of the sight is at right angles to the 
rule. And as this simple instrument does not give the 
inclinations, it may be best always to measure parallel or 
perpendicular to the horizon, and estimate the rest. In- 
deed, the contrivance must be considered only as a sub- 
stitute for the usual method of estimating, and may be 
principally useful to assist in acquiring a correct judg- 
ment in this respect. 
William Nicholson. 
