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.