An ophthalmologist using a tonometer measures the pressure in an eyeball to be 10 mmHg. What does this mean?

B: At sea-level on Earth, atmospheric pressure is about 760 mmHg. This value is taken to be “zero” and is often ignored by pressure measuring devices. When such devices show the pressure to be 10 mmHg on their gauge, this means that the actual pressure is 10 mmHg above atmospheric pressure. Hence total pressure is 760 + 10 mmHg.