A seconds pendulum is a pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 whose period is precisely two second

Second

The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

s; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz.

At standard gravity

Standard gravity

Standard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...

 its length is 0.994 m (39.1 in). This length was determined (in toise

Toise

A toise is a unit of measure for length, area and volume originating in pre-revolutionary France. In North America, it was used in colonial French establishments in early New France, French Louisiana , and Quebec...

s) by Marin Mersenne

Marin Mersenne

Marin Mersenne, Marin Mersennus or le Père Mersenne was a French theologian, philosopher, mathematician and music theorist, often referred to as the "father of acoustics"...

 in 1644. In 1660, the Royal Society

Royal Society

The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...

 proposed that it be the standard unit of length. In 1675 Tito Livio Burattini

Tito Livio Burattini

Tito Livio Burattini was an inventor, architect, Egyptologist, scientist, instrument-maker, traveller, engineer, and nobleman. He was born in Agordo, Italy, and studied in Padua and Venice...

 proposed that it be named the meter. In 1790, one year before the metre

Metre

The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

 was ultimately based on a quadrant of the Earth, Talleyrand proposed that the metre be the length of the seconds pendulum at a latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 of 45°. This option, with one-fifth of this length defining the foot, was also considered by Thomas Jefferson and others for redefining the yard in the United States shortly after gaining independence from the British Crown.

In 1670 the seconds pendulum was employed by William Clement in his improved version of the original pendulum clock

Pendulum clock

A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

 by Christian Huygens, creating the longcase clock

Longcase clock

A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...

 which could tick seconds.

In 1855 the British yard

Yard

A yard is a unit of length in several different systems including English units, Imperial units and United States customary units. It is equal to 3 feet or 36 inches...

was defined as a specified fraction of the length of the seconds pendulum.