an inoffensive joke

An artificial kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic the way real kidneys cleanse blood and eject impurities and surplus water as urine.

At preſent theſe trees are not very numerous, as the Turks make uſe of them to build the Grand Signior's caravelles, and cut down without ever planting.

On what do this underness and overness of supply depend?