Complex sentence: This is due to the fact that the change in air pressure is applied to the entire hydrometer flask; the submerged portion of the flask receives a transmitted force through the liquid, thus the flask receives no net force due to the change in air pressure.
Split into simpler sentences: This is due to the fact that the change in air pressure is applied to the entire hydrometer flask. The submerged portion of the flask receives a transmitted force through the liquid, thus the flask receives no net force due to the change in air pressure.