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The Encyclopedia of Horology

England · Denison's double three-legged form developed and applied c. 1850s, notably for the Great Clock of Westminster (completed 1854)

Gravity Escapement (Denison Double Three-Legged Form)

Partially verified

Solved a specific problem — maintaining pendulum regularity despite a variable and sometimes considerable load from external hands and weather — that earlier turret-clock escapements handled poorly.

A gravity escapement lets the pendulum receive its impulse from a small weighted arm that is itself raised by the going train and then allowed to fall onto the pendulum, isolating the pendulum's motion from direct variation in the driving force delivered by the train.

Denison's double three-legged version, developed for turret clocks exposed to variable wind loading on large exterior hands, became the standard escapement for precision tower clocks of significant size.

Background

The double three-legged gravity escapement was a design improved by Lord Grimthorpe, building upon a form first devised by a barrister named Bloxam. The mechanism addresses challenges faced by turret clocks where the wheel train experiences large variations in drive force caused by environmental loads like wind, snow, and ice on the exterior hands. In this system, the drive force from the wheel train does not directly impel the pendulum but instead resets the weights that provide the impulse.

The escapement features two gravity arms and two three-legged escape wheels, which operate in parallel planes to impact locking blocks on the respective arms. Lifting pins raise the gravity arms, providing a gain in potential energy that is delivered to the pendulum during each cycle.

Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.

How it works

  • Audible ticks are produced in the mechanism when the escape wheel teeth strike the locking blocks.

Later history

  • The Great Clock in the Elizabeth Tower at Westminster is a prominent example of this escapement's application.
  • The double three-legged gravity escapement became a standard for high-accuracy tower clocks.

How to verify this yourself

Denison's own published writings (including his Rudimentary Treatise on Clocks and Watches) describe the escapement's design rationale directly; any specific patent claim should be checked against The National Archives' historical patent index under Denison's name before citation.

Manufacturers known to have used it

  • E. J. Dent & Co. (Great Clock of Westminster)
  • many subsequent large turret-clock makers

Photographs

Gravity escapement
Gravity escapement — Hugh Hunt, Trinity College Cambridge, Public domain. Wikimedia Commons file page
Gravity escapement 2
Gravity escapement 2 — Hughhunt, Public domain. Wikimedia Commons file page
PSM V29 D195 Gravity clock escapement mechanism
PSM V29 D195 Gravity clock escapement mechanism — Unknown authorUnknown author, Public domain. Wikimedia Commons file page
PSM V29 D196 Gravity clock escapement mechanism
PSM V29 D196 Gravity clock escapement mechanism — Unknown authorUnknown author, Public domain. Wikimedia Commons file page
Wooden Clocks: Designing a new Gravity escapement
Wooden Clocks: Designing a new Gravity escapement — brianlawswoodenclocks.blogspot.com, Rights unverified — web image search result, review pending. Web page (brianlawswoodenclocks.blogspot.com)

Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.

Sources & references

  1. Escapement, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
  2. Edmund Beckett, 1st Baron Grimthorpe, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
  3. Denison, E. B. (Lord Grimthorpe), A Rudimentary Treatise on Clocks, Watches and Bells
  4. Antiquarian Horological Society, articles on the Denison gravity escapement

Where sources disagree, the disagreement is stated rather than resolved silently.