A sees Z commit what appears to A to be a murder. A, in the exercise, to the best of his judgment, exerted in good faith of the power which the law gives to all persons of apprehending murders in the act, seizes Z, in order to bring Z before the proper authorities. A has committed no offence, though it may turn out that Z was acting in selfdefence. 80. Accident in doing a lawful act: Nothing is an offence which is done by accident or misfortune, and without any criminal intention or knowledge in the doing of a lawful act in a lawful manner by lawful means and with proper care and caution. Illustration A is at work with a hatchet; the head flies off and kills a man who is standing by. Here if there was no want of proper caution on the part of A, his act is excusable and not an offence. 81. Act likely to cause harm, but done without criminal intent, and to prevent other harm: Nothing is an offence merely by reason of its being done with the knowledge that it is likely to cause harm, if it be done without any criminal intention to cause harm, and in good faith for the purpose of preventing or avoiding other harm to person or property. Explanation : It is a question of fact in such a case whether the harm to be prevented or avoided was of such a nature and so imminent as to justify or excuse the risk of doing the act with the knowledge that it was likely to cause harm. Illustrations (a) A, the captain of a steam vessel, suddenly and without any fault or negligence on his part, finds himself in such a position that, before he can stop his vessel, he must inevitably run down a boat B, with twenty or thirty passengers on board; unless he changes the course of his vessel, and that, by changing his course, he must incur risk of running down a boat C with only two passengers on board, which he may possibly clear Here, if A alters his course without any intention to run down the boat C and in good faith for the purpose of avoiding the danger to the passengers in the boat B, he is not guilty of an offence, though he may run down the boat C by doing an act which he knew was likely to cause that effect, if it be found as a matter of fact that the danger which he intended to avoid was such as to excuse him incurring the risk of running down C. (b)A, in a great fire, pulls down houses in order to prevent the conflagration from spreading. He does this with the intention in good faith of saving human life or property. Here, if it be found that the harm to be prevented was of such a nature and so imminent as to excuse A's act, A is not guilty of the offence. 82. Act of a child under seven years of age: Nothing is an offence, which is done by a child under seven years of age.