Andrewes, W.J.H. Clocks and Watches: The leap to precision in Macey, Samuel (1994). Encyclopedia of Time. Taylor & Francis. ISBN0-8153-0615-6. p.126, this cites a letter of December 11, but he may have meant the September 22 letter mentioned above.
"[Harrison's] escapement, called the 'grasshopper'... was of no practical value and need not be further described" Britten, Frederick James (1899). Old Clocks and Watches and their Makers. London: B. T. Batsford. p. 216.
Mundy, Oliver (June 2007). "Watch Escapements". The Watch Cabinet. Archived from the original on 2007-10-13. Retrieved 2007-10-18.
iinet.com
Hall, E. T. (1996). "The Littlemore Clock". NAWCC Chapter 161 - Horological Science. National Association of Watch and Clock Collectors. Archived from the original on 2007-12-24.
Hall, E. T. (1996). "The Littlemore Clock". NAWCC Chapter 161 - Horological Science. National Association of Watch and Clock Collectors. Archived from the original on 2007-12-24.
Beckett, Edmund; Cunynghame, Henry Hardinge (1911). "Watch" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 28 (11th ed.). Cambridge University Press. pp. 362–366, see end of page 363. The duplex escapement....