Royal Institution | 7th. June 1854.
My dear Tom
Lest I should forget it I will draw your attention to a very common and therefore very serious misconception under which Smithers appears to have laboured when he reported your lecture upon Barker’s mill.1 He speaks of the issuing water striking against the air and driving the arms round. Now the air you know has nothing to do in the matter and the instrument would rotate if it were placed in a vacuum. Of course the force of the issuing jet, whatever that may be, is exerted in the opposite direction to the jet’s motion and this it is which produces the motion.
Thine | John
RI MS JT/1/HTYP/343
LT Typescript Only
your lecture upon Barker’s mill: H. Smithers, ‘Local Information’, The Queenwood Observer, 2:4 (31 May 1854), p. 25. Barker’s mill was ‘a mechanical contrivance for producing rotary motion, consisting of a hollow vertical revolving axis with two (or more) horizontal arms fitted into its lower end’ invented during the late seventeenth century (OED).
Please cite as “Tyndall0936,” in Ɛpsilon: The John Tyndall Collection accessed on