The first instrument, called the Suspended Plane, was devised to illustrate an unfamiliar application of a known principle. I call the application unfamiliar because distinguished physicists have held, for instance, that a bird (which obviously expends a certain amount of muscular effort in simply hovering in the air) must expend in flight all the effort required for hovering, together with so much additional energy as is required to overcome the resistance of the air to its horizontal motion, so that the energy expended increases with the velocity attained, ^[*] while the consideration of the action of the suspended plane indicates, if it do not demonstrate, that the opposite view is the true one, and thus serves as a useful introduction to the demonstrative experiments I have spoken of as coming later.

The suspended plane (plate IV) consists of a thin brass plane one foot square, weighing two pounds, hung vertically by a spring from a surrounding frame. Eight delicate friction rollers AA ?, BB ? enable the plane to move freely along the frame, but prevent any twisting or lateral motion, the use of the guide-frame being to prevent the plane from so flouncing under irregular air currents that its pull cannot be measured. The guide-frame carrying the plane turns symmetrically about an axis, CC ?, so that the gravity-moment about the axis is simply the weight of the plane on a lever arm measured from its center. The axis CC ? rests...