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PHILOSOPHICAL SOCIETY OF WASHINGTON. 
Mr. G. K. Gilbert presented a communication entitled 
GRAPHIC METHODS IN RESEARCH. 
[Abstract.] 
An algebraic equation between two variables is the equivalent of 
a plane curve, or, more strictly, of a line lying in a plane. An 
equation containing three variables is the equivalent of a surface. 
If in such an equation one of the variables be assumed equal to 1, 2, 
3, etc., successively, there result a series of equations involving the 
other two variables. Each of these equations may be represented 
by a line in a plane, and the system of lines thus produced is the 
representative of the original equation between three variables. The 
single curve may be called a nomogram, the system of curves an 
iso gram. 
In the simplest use of the graphic method in research the simul¬ 
taneous quantitative observations of two phenomena are represented 
on cross-section paper by a dot, a series of observations are repre¬ 
sented by a system of dots, and a line is drawn through or among 
these dots. This line expresses the law of the relation between the 
two phenomena and is a nomogram. 
When simultaneous observations are made of three phenomena, 
two of the observations are expressed by a dot on section paper and 
the third by a number attached to the dot. All the observations 
having been represented by such numbered dots, a system of lines 
is drawn over the area occupied by the dots, each line representing 
an integral value of one of the variable phenomena, and being 
drawn so as to pass through the dots marked with the correspond¬ 
ing number. An isogram is thus produced without recourse to the 
algebraic equation. 
In the compound nomogram, two or more curves are drawn on the 
same sheet, and with one system of ordinates in common. Each of 
these curves represents an equation with two variables, one vari¬ 
able being common to all the equations. By this means two or 
more variable phenomena are compared with each other through 
the mediation of another phenomenon with which they are related. 
In the compound isogram two or more isograms are drawn on the 
same sheet. Each isogram is the equivalent of an equation between 
three variables, two variables being common to all the isograms. 
