Cavendish weighed the Earth by hiding from his own experiment

By the late 18th century, scientists understood that gravity acted between all masses, not just between the Earth and objects on it, but no one had directly measured the gravitational pull between two ordinary objects in a laboratory — the force involved was almost immeasurably small at any scale a room could hold. Getting a reliable number for the Earth's density, and from it the strength of gravity itself, meant detecting an attraction between two lead spheres that was a tiny fraction of the force needed to lift a feather.

At that scale of sensitivity, the equipment itself became the least of the problem: air currents from someone walking past, or even the heat radiating from an experimenter's own body standing near the apparatus, could exert more force on a sensitive torsion balance than the gravity it was built to detect. Simply building a more sensitive instrument wouldn't help if the person reading it kept contaminating the measurement just by being in the room.

Sealed the torsion balance in a shed and read the deflection through a telescope from an adjoining room, so the observer's heat and breath never touched the measurement.

Sealing the apparatus in a shed blocked air currents and temperature changes from the observer.

Reading through a telescope let Cavendish see the result without entering the room.

Removing the observer's heat and breath eliminated the largest source of error.

The torsion balance then measured only the gravitational force, yielding a result within 1% of today's value.

When the observer's presence disturbs the measurement, separate observation from the experiment physically or temporally.

Cavendish's result put the Earth's average density at 5.448 times that of water, and the gravitational constant derived from his data — 6.74×10⁻¹¹ m³kg⁻¹s⁻² in modern units — came within about 1% of the value accepted today. The Cavendish experiment is treated as a landmark in the history of experimental physics, both for the first laboratory measurement of gravity between ordinary masses and for the observational discipline it demonstrated: isolating the measuring instrument from the observer's own physical presence, a principle that runs through precision measurement to this day.