15th Nov. 1861.
My dear Mr Stokes
Would you kindly favour me with your opinion1 on the following point.
A flame plays against one of the vertical faces of an exhausted cubical vessel; being competent to keep that face at a temperature of 200°.
The opposite face of the cube is kept at the constant temperature of 0°.
Half an atmosphere of air is now admitted into the cube; and convection sets in between the hot parts & the cold ones.
The amount of chilling of the hot face is capable of accurate measurement: let it be measured & noted.
An additional half atmosphere is now admitted in to the cube, making the total amount there an atmosphere. The chilling is neither augmented nor diminished by the additional half atmosphere.
Now inasmuch as we have in the case of the whole atmosphere double the number of particles loading themselves with heat & carrying it off, and inasmuch as twice the number only do the same work as once the number am I not justified in concluding that the average velocity of the carrying particles in the case of the half atmosphere is double their velocity in case of the whole atmosphere.
Yours very sincerely | John Tyndall
RI MS JT/1/T/1396
RI MS JT/1/TYP/4/1476
favour me with your opinion: for Stokes’ reply, see letter 1829.
Please cite as “Tyndall1828,” in Ɛpsilon: The John Tyndall Collection accessed on