(6) of section 18 A so far as it is material for our purpose is in these terms "Where in any year an assessee has paid tax under sub section (2) or sub section (3) on the basis of his own estimate, and the tax so paid is less than eighty per cent of the tax determined on the basis of the regular assessment, so far as such tax relates to income to which the provisions of section 18 do not apply and so far as it it not due to variations in the rates of tax made by the Finance Act enacted for the year for which the regular assessment is made, simple interest at the rate of six per cent per annum from the 1st day of January in the financial 677 year in which the tax was paid up to the date of the said regular assessment shall be payable by the assessee upon the amount by which the tax so paid falls short of the said eighty per cent xx xx xx xx xx xx xx xx XX.