The omission to do something which a reasonable man, guided by those considerations which ordinarily reg-. ulate the conduct of human affairs, would do, or doing something which a prudent and reasonable man would not do. It- must be determined in all cases by reference to the situation and knowledge of the partles and all tbe attendant drcumstauces. Nitro-Glycerin Case, 15 wall. 536, 21 L. Ed. 206; Blythe v. Birmingham waterworks Co., 11 Exch. 784