To those who have been engaged in the construction of mining and metallurgical plants during the war and post-war periods, it is quite unnecessary to point out the large increases and violent fluctuations in prices of basic commodities and the steady and persistent rise in labour costs. It may, however, be of interest to present these facts in graphic form, and survey the relative heights of the various maxima and the lead and lag of individual waves. Since Descartes demonstrated for us moderns the analogy between geometric form and algebraic statements, the two-dimensional graph has found widespread application. Mathematicians have developed for our information, if not for our confusion, the triangular diagram and the solid three-dimensional figures, which have proved so useful, at least in the hands of the metallurgists at Port Pirie. The model (Plate 1.) which is exhibited here does not pretend however, to indicate the connection between any three inter-related variables, and it is not proposed to burden members with any abstruse mathematical expression for the irregular surface of the blocks. While time is an ordinate common to all sections of the model, each illustrates separately the variation in cost of some particular quantity between the years 1914 and 1922. It is an easy matter to record such individual variations, but it would be an impossible task to state any equation describing accurately the dependence ot the prices, say, of hardwood or of galvanized iron upon that of labour at any particular point of time. The cost of living curve, which has attained such prominence, if not notoriety, in the last few years, is the result of an attempt to show the mass-effect of the components. It is the difficulty found in "assigning the correct weight to the separate items which makes the aggregate curve somewhat misleading at times.Turning to the model, it will be noticed that each section consists of half a dozen pegs.. The first in each row,that coloured red, represents the cost of some particular quantity, such as bricks, timber or coal, at Broken Hill in 1914. The next peg, that coloured blue, represents proportionately prices for that quantity in Hobart during...