Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/437

Rh “Say thus:—Divide the wave train into lengths each containing unit mass. The time taken by each of these lengths to pass a given point is proportional to the length. Also the time during which, etc. ... or something of the sort. But it appears to me that this is not the whole story, but that the motion docs communicate momentum. If the velocity forward is $$u\ \longrightarrow ,$$ momentum $$\rho\ u \times u = \rho\ u^2$$ crosses the plane $$\longrightarrow .$$ If the velocity is $$u\ \longleftarrow ,$$ momentum $$\rho\ u^2$$ crosses the plane $$\longleftarrow ,$$ and both of these give an addition of momentum $$\longrightarrow$$ to the region on the forward side of the plane.”

For the purpose of reference p. 3 of the author's original MS. is given in the accompanying footnote, the paragraph marked by Prof. Poynting being italicised. The initial and final paragraphs are completed as on pp. 2 and 4 of the MS.