He explains that ... the time taken for the object to fall is "PROPOTIONAL TO\[\alpha\]" So many factors, but for that instance, we take three factors
1. height at which the object is falling from so he takes H
2. M. mass of the object, which is falling (this is in relation to the famous question, is it the feather or the apple which reaches ground first from the same height).
3. Gravitational acceleration the more g is, fall will be faster (Value of g varies in various points of earth).
So he took the three factors,
Time \[\alpha\] Height,
T \[\alpha\] Mass,
T \[\alpha\] g ..
so, T \[\alpha\] H M and g.