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POPULAR SCIENCE REVIEW. 
by accurate measurement, than wliicb nothing can be easier. 
There is, however, one precaution that must be taken, and this 
is rendered necessary from the fact that the pen of the sphygj- 
mograph lever moves in part of a circle, a large one no doubt, 
and not in a straight line. Consequently, if the commence- 
ment of the primary and secondary rises in each beat are at a 
different level, which is generally the case, it is impossible to 
judge of the length of the systole, which, as far as it affects the 
pulse, is the interval between these points, without projecting- 
them on the same horizontal line. If this were done by simply 
drawing vertical lines on the tracing and then making the 
measurements, imperfect results would be obtained; so it is 
necessary to make the lines curved, just the same as if the lever 
of the instrument had drawn them whilst the watchwork was 
not moving. To draw these lines with the tracing on the 
sphygmograph itself, with its own lever, is difficult, and wears 
up the watchwork unnecessarily. It is much better, after 
having varnished the trace, to scratch the lines on it with a 
needle, which is so fastened to strings of the same length as the 
lever that it can produce curves of exactly the same radius. 
The following sketch gives an idea of the apparatus I am 
THE CTJRYE-CONSTEUCTING INSTRUMENT fOR SPHYGMOGRAPH TRACINGS. 
(Half the described size.) 
accustomed to employ. It is extremely simple, and requires 
no skill in manufacture. A thin slab of wood (a), 8 inches by 
4, has another straight strip of the same material, 6 inches 
long (b), nailed along one of its longer sides to form a ridge 
against which the tracing may rest. A small screw (c) is then 
fixed in a line continuous with this piece of wood, in such a 
position that it holds exactly the same relation to the ridge 
formed between it and the bigger slab that the moving, trace- 
