In this form of lever the power is applied between the fulcrum and the weight to be raised.
Although the third order of lever is particularly inefficient when the fulcrum is rigid and immobile, it possesses singular advantages when these conditions are reversed, that is, when the fulcrum, as happens with the air, is elastic and yielding.
The line j oining the end knife edges must be exactly bisected by a line drawn perpendicular to it from the fulcrum knife-edge.
It consists of a steelyard mounted on a fulcrum; one arm carries at its extremity a heavy bob and pointer, the latter moving along a scale affixed to the stand and serving to indicate when the beam is in its standard position.
The balloon has no hold upon the air, and this consequently cannot be employed as a fulcrum for regulating its course.