There is no special rule for representing physical quantities such as electric force, potential, temperature, etc.; we may draw the isotherms as straight lines, ellipses, spheres, according to convenience of illustration. But there are certain physical quantities (i.e.\ results of operations and calculations) which have a natural graphical representation; we habitually think of them graphically, and are almost unconscious that there is anything conventional in the way we represent them. For example, measured distances and directions are instinctively conceived by us graphically; and the space in which we represent them is for us \emph{actual space}. These quantities are not in their intrinsic nature dissimilar from other physical quantities which are not habitually represented geometrically. If we eliminated the human element (or should we not say, the pre-human element?)\ in natural knowledge the device of graphical representation of the results of measures or estimates of distance would appear just as artificial as the graphical representation of thermometer readings. We cannot predict that a superhuman intelligence would conceive of distance in the way we conceive it; he would perhaps admit that our device of mentally \PageSep{197} plotting the results of a survey in a three-dimensional space is ingenious and scientifically helpful, but it would not occur to him that this space was more \emph{actual} than the $pv$~space of an indicator-diagram. In our previous work we have studied this unsophisticated graphical representation of certain physical quantities, under the name Natural Geometry; we have slightly extended the idea by the addition of a fourth dimension to include time; and we have found that not only the quantities ordinarily regarded as geometrical but also mechanical quantities, such as force, density, energy, are fully represented in this natural geometry. For example the energy-tensor was found to be made up of the Gaussian curvatures of sections of actual space-time~\Eq{(65.72)}. But the electromagnetic quantities introduced in the preceding chapter have not as yet been graphically represented; the vector~$\kappa_{\mu}$ was supposed to exist \emph{in} actual space, not to be the measure of any property \emph{of} actual space. Thus up to the present the geometrisation of physics is not complete.