2. Standards of the Linear Measures
Tables of measures, from the earliest, about 1500, down to quite recent times, usually began by stating that ‘Three barley-corns make an inch’ or that ‘Geographical measures begin at a barley-corn and increase upward to a league,’ &c.
King David I of Scotland (c. 1150) is credited with the pronouncement that the Scots inch was to be the mean measure of ‘the thowmys of iij men, that is to say an mekill man and a man of messurabil statur and of a lytell man. The thoums are to be messurit at the rut of the nayll.’ But no more in Scotland than in England, or elsewhere, has the inch ever been anything but a division of the foot.
A standard of the English foot was fixed in Old St. Paul’s Church, London, and was known as Paul’s foot, all measures being referred to the standard ‘qui insculpitur super basim columpnæ in ecclesia Sancti Pauli.’ In 1273 a deed gave the measurement of land ‘according to the iron ell [yard] of the King’s palace.’
The present standard yard is a bronze bar kept in London, the length of which agrees exactly with the yard, still extant, of Tudor times. A set of standard measures of length is fixed along the base of the northern wall of Trafalgar Square,[[14]] and another set is in the flooring of the Guildhall. Sets are also fixed to public buildings in several chief towns of the United Kingdom.
As metal rods vary in length according to temperature, comparisons with a standard measure should be made at the normal temperature of 62°. But there is an alloy of steel and nickel (42 per cent.), named Invar, which is not perceptibly affected by temperature.
A pendulum beating seconds at sea-level and at normal temperature measures 39·1393 inches at Greenwich (Act of Parliament, 1824). This length varies in different places from the variations of gravity due to the ellipticity of the earth and local causes of deviation.