for the effects of temperature. Atmospheric drag The viscosity of the air through which the pendulum swings will vary with atmospheric pressure, humidity, and temperature. This drag also requires power that could otherwise be applied to extending the time between windings. Traditionally the pendulum bob is made with a narrow streamlined lens shape to reduce air drag, which is where most of the driving power goes in a quality clock. In the late 19th century and early 20th century, pendulums for precision regulator clocks in astronomical observatories were often operated in a chamber that had been pumped to a low pressure to reduce drag and make the pendulum's operation even more accurate by avoiding changes in atmospheric pressure. Fine adjustment of the rate of the clock could be made by slight changes to the internal pressure in the sealed housing. Leveling and "beat" To keep time accurately, pendulum clocks must be level. If they are not, the pendulum swings more to one side than the other, upsetting the symmetrical operation of the escapement. This condition can often be heard audibly in the ticking sound of the clock. The ticks or "beats" should be at precisely equally spaced intervals to give a sound of, "tick...tock...tick...tock"; if they are not, and have the sound "tick-tock...tick-tock..." the clock is out of beat and needs to be leveled. This problem can easily cause the clock to stop working, and is one of the most common reasons for service calls. A spirit level or watch timing machine can achieve a higher accuracy than relying on the sound of the beat; precision regulators often have a built-in spirit level for the task. Older freestanding clocks often have feet with adjustable screws to level them, more recent ones have a leveling adjustment in the movement. Some modern pendulum clocks have 'auto-beat' or 'self-regulating beat adjustment' devices, and don't need this adjustment. Local gravity Since the pendulum rate will increase with an increase in gravity, and local gravitational acceleration varies with latitude and elevation on Earth, the highest precision pendulum clocks must be readjusted to keep time after a move. For example, a pendulum clock moved from sea level to will lose 16 seconds per day. With the most accurate pendulum clocks, even moving the clock to the top of a tall building would cause it to lose measurable time due to lower gravity. The local gravity also