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POPULAR SCIENCE REVIEW. 
communicate with one another. Both this and the knife-edge 
instrument have advantages of their own ; but it seems to me 
that in the long run the older one will be found the more 
generally applicable, and less likely to get out of order. 
The recording paper should be highly glazed, like old- 
fashioned visiting-cards, to reduce the friction to the least pos- 
sible amount ; and further to do so, it is much better, instead of 
using a split pen (with two points necessarily) and ink, to 
smoke the paper and use a simple steel point as the pen. The 
paper is smoked by doubling the ends of the strip round the 
brass plate on which it is supported, and passing it over the 
flame of an ordinary composite candle until it is dark, but not 
black. After the tracing has been taken it should be thinly 
varnished, by pouring an ordinary photograph spirit varnish over 
it, in the same way that collodion is made to flow over a glass 
plate. The varnish darkens the smoke film considerably. If it 
is put on thick it is very apt to crack off when the paper is 
bent. 
To take a trace, the screw which presses the pulse-spring 
should be raised so as to laxen it. Some slight mark should be 
made by beginners on the bare arm, where the pulse is felt to 
beat most strongly. The ivory pulse-pad should then be placed 
over the spot, care being taken that is not pressed to one side 
or the other ; and the whole instrument strapped down. After 
this, the screw pressing the pulse-spring should be turned until 
the artery is considerably compressed, and the lever adjusted to 
beat in its proper place just opposite where the recording paper 
is to run. A fully doubled handkerchief, or something of about 
that thickness, should be placed between the arm and the part 
of the instrument on which the watchwork is fixed, to support 
it. The moving train, with the paper already smoked fixed on 
it, is then adjusted to its place, after which, the rate at which 
the pulse is beating is found by a watch in the ordinary way. 
All is then ready for the trace being taken. 
As the watchwork drives the recording paper in ten seconds 
rvhen the instrument is level , if such has been the case, the 
pulse-rate may be calculated indirectly ; but to insure greater 
accuracy, it is best always to check the result by direct 
observation. 
The next point for consideration is the value of the tracings 
taken with Marey’s instrument ; what more they teach than can 
be learnt by simply applying the finger on the pulse ; and how 
the language of the instrument, as the interpretation of its 
minutiae may be termed, is to be understood. 
As the sphygmograph can only be applied to the arteries 
which supply the various tissues of the body, and not to those 
which run to the lungs — that is, to the systemic and not to 
