Depreciation rates for machinery and plant revised to vary by installation date and commercial purpose. Amendment substitutes clauses in section 33 to set depreciation on machinery or plant by actual cost: twenty-five percent for installations before 1 April 1961; for installations after 31 March 1961, thirty-five percent where installed after 31 March 1963 and before 1 April 1966 for coal mining, and twenty percent in any other post-31 March 1961 case.