(1) A device applied to English harpsichords in the second half of the 18th century by means of which a single pedal could be made to control two or more separate registers, overriding their individual handstops (see Pedal (4)). On single-manual harpsichords, when the pedal was depressed the 4' register and one of the 8' registers were withdrawn, and when the pedal was released both registers were re-engaged. Moreover, when the pedal was depressed slowly, the 4' register was withdrawn before the 8', and when the pedal was released slowly, the 8' register was re-engaged before the 4', thereby permitting the harpsichordist to produce fairly smooth diminuendos and crescendos. On two-manual harpsichords, the machine stop had an identical effect on the registers available on the lower manual but, in addition, when the pedal was depressed, the close-plucking lute stop was engaged in place of the front (‘dogleg’) 8' register on the upper manual. Both the single-manual and double-manual machine stops could be disengaged when desired, returning the harpsichord to normal handstop operation. It is thought that the machine stop was invented by Burkat Shudi, perhaps specifically for Frederick the Great’s harpsichords.
(2) A device with a similar purpose applied to chamber organs sometimes called a ‘shifting movement’. An extra slider could cancel out the higher-pitched stops when a pedal was depressed, even though their knobs remained drawn, and restore them when the pedal was released. The machine stop was common on English chamber organs from the mid-18th century onwards, and was used in echo passages and in pieces having short soft sections or interludes.
BoalchM
M. Wilson: The English Chamber Organ (Oxford, 1968)
E.M. Ripin: ‘Expressive Devices Applied to the Eighteenth-Century Harpsichord’, Organ Yearbook, i (1970), 64
K. Mobbs and A. Mackenzie of Ord: ‘The “Machine Stop” and its Potential on Full-Specification One-Manual Harpsichords Made by Thomas Culliford’, GSJ, xlvii (1994), 33–46
EDWIN M. RIPIN (1), BARBARA OWEN (2)