An agent, unless specially forbidden by his principal to do so, can delegate his powers to another person in any of the following cases, and in no others:
1.When the act to be done is purely mechanical;
2.When it is such as the agent cannot himself, and the sub-agent can lawfully perform;
3.When it is the usage of the place to delegate such powers; or,
4.When such delegation is specially authorized by the principal.