Defining the line of motion and the line of gravity
Spiritual versus real force at the lever r S t v; pulley and balance sketches
Leonardo sets down two definitions central to his statics: the 'line of motion,' which runs between a weight and the cause that moves it, and the 'line of gravity,' which sustains a weight along its path with a fixed length. A further note distinguishes the lever r S t of the 'spiritual' (potential) force from the line r t v of the 'real' force. Small pulley and balance sketches carry the loads (16, 8, 4) that illustrate the definitions.
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The line of motion defined
The line of motion is that which extends between the weight and the cause that moves it. It is the geometric line along which a moving load and its driving cause act.
The line of gravity defined
The line of gravity is that which, keeping a fixed length, sustains the weight throughout its motions. It is the line along which a load's heaviness is borne.
Spiritual and real force: r S t and r t v
r S t is the lever of the motion and of the spiritual force, while r t v is the line of the motion and of the real force. Leonardo separates the virtual (potential) action from the actual line of the load.
Pulley and balance diagrams
The definitions are set beside small frames in which cords run over pulleys to weights marked 16, 8 and 4. The sketches show the lines of motion and gravity in concrete balance arrangements.