188. An agent having an authority to do an act has authority to do every lawful thing which is necessary in order to do such act. An agent having an authority to carry on a business has authority to do every lawful thing necessary for the purpose, or usually done in the course of conducting such business.Illustrations (a) A is employed by B, residing in London, to recover at ²[Chittagong] a debt due to B. A may adopt any legal process necessary for the purpose of recovering the debt, and may give a valid discharge for the same. (b) A constitutes B his agent to carry on his business of a ship-builder. B may purchase timber and other materials, and hire workmen, for the purposes of carrying on the business.