Marrison, Warren A. «The Evolution of the Quartz Crystal Clock» (en anglès). The Bell System Technical Journal, Vol. XXVII p. 510-588. IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society, 1948. Arxivat de l'original el 2011-07-17. [Consulta: 29 octubre 2011].
Plini el Vell. «CHAP. 15. (10.)—THE OBELISK WHICH SERVES AS A DIAL IN THE CAMPUS MARTIUS.». A: John Bostock, M.D., F.R.S., H.T. Riley, Esq., B.A., Ed.. The Natural History (en anglès) [Consulta: 22 agost 2011]. «For nearly the last thirty years, however, the observations derived from this dial have been found not to agree: whether it is that the sun itself has changed its course in consequence of some derangement of the heavenly system; or whether that the whole earth has been in some degree displaced from its centre, a thing that, I have heard say, has been. remarked in other places as well; or whether that some earthquake, confined to this city only, has wrenched the dial from its original position; or whether it is that in consequence of the inundations of the Tiber, the foundations of the mass have subsided, in spite of the general assertion that they are sunk as deep into the earth as the obelisk erected upon them is high.»
warrencenter.com
«The History of Henry Warren» (en anglès). Warren Conference Center & Inn. Arxivat de l'original el 2010-08-10. [Consulta: 29 octubre 2011].
Schraven, Thomas. «The Hipp chronoscope» (PDF) (en anglès) p. 7. Arxivat de l'original el 2012-03-08. [Consulta: 29 octubre 2011].
«The History of Henry Warren» (en anglès). Warren Conference Center & Inn. Arxivat de l'original el 2010-08-10. [Consulta: 29 octubre 2011].
Marrison, Warren A. «The Evolution of the Quartz Crystal Clock» (en anglès). The Bell System Technical Journal, Vol. XXVII p. 510-588. IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society, 1948. Arxivat de l'original el 2011-07-17. [Consulta: 29 octubre 2011].
Valls, Xavier «El rellotge al llarg del temps». Sàpiens [Barcelona], núm. 79, 5-2009, p. 46-49. ISSN: 1695-2014.
Cotterell, Brian; Kamminga, Johan. Mechanics of pre-industrial technology: An introduction to the mechanics of ancient and traditional material culture. Cambridge University Press, 1990. ISBN 0521428718. OCLC18520966., pp. 59–61