Ron Smith teaches this Yale lecture from The Atmosphere, the Ocean and Environmental Change (GG 140), covering how the hydrostatic law describes the weight of a fluid column and the pressure at a given point. After a brief recap of planet temperature, Smith derives the differential form of the hydrostatic balance equation and shows how it can estimate the total mass of CO2 in the atmosphere. A classroom experiment measures the pressure difference between the ground floor and the twelfth floor of Klein Biology Tower, comparing the predicted value against direct barometer readings. The lecture closes by extending the same balance law to pressure changes with depth in the ocean, linking atmospheric and oceanic physics through one governing equation. Recorded in Fall 2011 as part of Yale's Open Courses series.