'' (1) The President of the Union shall, by notification, declare the rates at which dues leviable under this Act shall be paid, and (a) may cause such rates to vary in any or all of the following cases — (i) according to the class of the goods. (ii) according as the goods are exported from or imported into a port. (iii) according to the place to which the goods are to be exported or from which they are imported, and (iv) according to the class of ve:sel ; and (b) may exempt specified classes of vessels or goods from the operation of this Act...''