When there is a question whether a particular act was done, the existence of any course of business, according to which it naturally would have been done, is a relevant fact. Illustrations. (a) The question is, whether a particular letter was dispatched. The facts that it was the ordinary course of business for all letters put in a certain place to be carried to the post, and that particular letter was put in that place are relevant. (b) The question is, whether a particular letter reached A. The facts that it was posted in due course, and was not returned through the Return Letter Office, are relevant.