(c) A instigates B to set fire to a dwelling- house. B, in consequence of the unsoundness of his mind, being incapable of knowing the nature of the act, or that he is doing what is wrong or contrary to law, sets fire to the house in consequence of A' s instigation. B has committed no offence, but A is guilty of abetting the offence of setting fire to a dwelling- house, and is liable to the punishment provided for that offence.