These experiments, though they have continued over a considerable period of time, are, unfortunately, by no means conclusive in results. I may as well explain how and why I conducted them. Actual trials of aërial propellers have not very often been made, or perhaps I should say that the results of such trials have not often been published. This latter may be for two reasons : 1st, that inventors wish to keep the results of their investigations to themselves, which, though somewhat selfish, is but natural. 2nd. The results may be such that the experimenters do not think them worth recording. I have considered whether I should not comply with this second reason, and simply look upon the results of my work as negative and not worth recording.