In a body propelled by planes in motion, the centre of gravity of the weight of body is assumed to be sustained on a uniformly level path, to be sustained in the manner of a weight at rest, without momentum in the direction of support; where as the propelling planes will move on an undulating air path.

England is not the only country that can boast of an Aëronautical Society, for France possesses one which is worthy of the land where Aëronautics first saw the dawn. In 1863 all Europe was roused and interested in the solution of the problem of aërial navigation, by the energetic appeals of MM. Nadar and La Landelle. An Aëronautical Society was founded, and for a year or two flourished; but as nothing practical was forthcoming, it soon languished and died. The present Society was founded in 1868 by Dr. Hureau de Villeneuve, to whom, in a great measure, its present flourishing state is due. This energetic gentleman also started a monthly magazine, called l'Aëronaute, which has continued to appear regularly ever since April, 1868, and is the only instance in which a journal specially devoted to aeronautical science has been a continued success.