Signature essential to liability

No person is liable as drawer, endorser or acceptor of a bill who has not signed it as such, Provided that, 19 (a) where a person signs a bill in a trade or assumed name, he is liable thereon as if he had signed it in his own name; (b) the signature of the name of a firm is equivalent to the signature by the person so signing of the names of all persons liable as partners in that firm.