246 C H A P T E R 11 on the slider and measure 381 mm. Scale A is the tank capac- ity, calibrated from 15 000 to 50 000 cubic feet. Scales B and C are international specific gravity values from 0.6 to 1.08. These are reversed logarithmic scales running from 33.292 to 59.972 with markings giving the specific gravities. The reversal of these scales is exactly as in figure 11.12. The D scale is marked in American petroleum industry (API) specific gravity values, and E is calibrated in tons for values between 450 and 1500 tons per tank. The cursor has three lines, marked I, II and III from right to left. Lines II and III correspond to multiplication by 1.015 and 1.12 and the three lines allow calculations in long or short tons or in metric tonnes, respectively. The cursor is also fitted with a clamping device that holds it in one place relative to the slider. Loading problems always involve oil of a fixed specific gravity so being able to accurately fix the cursor has obvious advantages. Essentially the moving cursor becomes a fixed index point. To perform a calculation, the cursor is fixed at a specific grav- ity and then the slider, with fixed cursor, is moved to a corre- sponding volume on the top scale A. The weight is read directly off scale E using the appropriate index point. This is exactly as described in section 11.5 above. The rule can be used for tanks calibrated in water tons and cubic metres (metric water tonnes) by using lines II and III on the cursor. Weight to volume cal- culations are straightforward using the reverse process. Lastly, API specific gravities can be converted to International specific gravity units by allowing the cursor to run freely and reading across scales C and D. United Kingdom Patent 919063 was granted to the inventor Guy William Farrier Sangwin, the complete specification having been published on 20 February 1963 (Sangwin 1963). The cal- culations for the scale and the printing of the plates took three years to complete. The prototype rule, with scales printed onto plastic and laminated onto a wooden base, survives although the leftmost 150 mm of scales A and E have been badly dam- aged by water (the rule was rescued from a garage). This is