This quantity was measured in experiments by Agnes Pockels using a homemade sliding trough (troff) that evolved into the Langmuir-Blodgett trough. This quantity is proportional to the force needed to lift a Wilhelmy plate or a du Nouy (10[3])ring. Three values of this quantity are related to the cosine of the (-5[1])contact angle by Young's relation, while this quantity times the subtracted reciprocals of two radii of curvature gives the pressure difference in the Young-Laplace (luh-PLOSS) equation. Mass is transferred along a gradient in this quantity in the (10[1])Marangoni effect, creating ■END■