SYMBOLS OF RELATION. 255
For pressure-organs we require to distinguish between gases and liquids. We may use the Greek letters X and 7 for this purpose, and thus have :
$ A liquid pressure-organ, water, etc. Q^ gaseous pressure-organ, gas, air, steam, etc. In certain cases the pressure-organ consists of more or less round grains, which may with sufficient accuracy be taken as spherical, and therefore we have
Q s or more exactly Q~ s for a pressure-organ consisting of
more or less globular portions.
We shall form and use further compound symbols as we have occasion.
f>7. Symbols of Relation.
Of the relations which one element in a chain can have to another the most important are those of pairing and linkage. The first we may indicate by a comma. C, C will thus stand for two cylinders rolling together, C + ) C + would be used if both were full, C~,C+ if one were full and one open. We shall always pre- suppose that the comma indicates both the possibility and the existence of correct pairing. Thus we shall not require any sign beyond O + , C+ to show that the axes of the cylinders are parallel, while C~,C~ is incorrect, for it is impossible to form a kinematic pair from two open cylinders.
Linkage will be denoted by a dot or dotted line. C + C ~*~
for instance is a link having two full cylinders for the elements
which it connects, C~ C~ a link connecting two open
cylinders or eyes.
The fixing of a link may be indicated by underlining the dotted
lines. P + C+ for instance stands for a fixed link connecting
a full prism and a full cylinder.
It may occasionally be necessary to indicate that a link is elastic, namely that it is a spring, in which case a wavy line may be placed over the dotted one ."?7T. 7 . .
A number of other signs are partly the same as the common arithmetical signs, and partly based upon them. They are as follow :
