'The elements of the defence are conveniently stated in Halsbury's Laws of England ... . There it was said that: "In order to establish the defence, the plaintiff must be shown not only to have perceived the existence of danger ... but also that he fully appreciated it and voluntarily accepted the risk. The question whether the plaintiff's acceptance of the risk was voluntary is generally a question of fact, and the answer to it may be inferred from his conduct in the circumstances. The inference may more readily be drawn in cases where it is proved that the plaintiff knew of the danger and comprehended it as, for example, when the danger was apparent, or proper warning was given of it, and there was nothing to show that he was obliged to incur it, than in cases where he acknowledged that there was danger but not for comprehension of its extent, or where, while taking an ordinary and reasonable course, he had not an adequate opportunity of electing whether he would accept the risk or not."'