Up to the present time the problem of stability has received very inadequate attention in connection with aviation. From the point of view of the practical aviator, this is, perhaps, little to be wondered at. It would scarcely be possible for him to devote months of concentrated attention to long and laborious stability investigations when it is no exaggeration to say that very frequently his success or failure depends, above all things, on the names of the towns at which he starts and lands. If a prize is offered for a flight from Folkestone to Flushing, it is useless for him to fly from Harwich to the Hook, even on a much more stable machine than that used by the winner of the prize.<br><br>It is hoped that the publication of this memoir will lead to aeroplane stability being made the subject of much more continuous study and investigation than has been possible in the past. A general abstract of the present investigation is contained in the introduction, which may be read with advantage before proceeding to details of a more mathematical character.