Reciprocation: Though many people would agree that reciprocation shouldn't necessarily dictate how much money you gift at a wedding, it definitely plays into the decision for most people.
In the particular case where the motion is of the kind known as simple harmonic the disturbing force on the frame due to the reciprocation of the weight is equal to the component of the centrifugal force in the line of stroke due to a weight equal to the reciprocated weight supposed concentrated at the crank pin.
In practice reciprocation is usually effected through a connecting rod, as in the case of steam engines.
Although this method balances the pistons in the horizontal plane, and thus allows the pull of the engine on the train to be exerted without the variation due to the reciprocation of the pistons, yet the force balanced horizontally is introduced vertically and appears as a variation of pressure on the rail.
A focus or directrix is equal to two conditions; hence such problems as: given a focus and three points; a focus, two points and one tangent; and a focus, one point and two tangents are soluble (very conveniently by employing the principle of reciprocation).