1899-1900.] Lord Kelvin on the Motion in an Elastic Solid. 229 
§11. We have thus in § 10 solved one of the two chief 
kinematic questions presented by our problem : — to find the orbit 
of a particle of ether as disturbed by the moving atom, relatively 
to the surrounding ether supposed fixed. The other question, to 
find the path traced through the atom supposed fixed while, 
through all space outside the atom, the ether is supposed to move 
uniformly in parallel lines, is easily solved, as follows : — Going 
back to fig. 3, suppose now that instead of, as in § 10, the atom 
moving from right to left with velocity *1 and the ether outside 
it at rest, the atom is at rest and the ether outside it is moving 
from left to right with velocity *1. Let '2, '3, '4, '5, '6, '7, '8, '9, 
0, '1, '2, '3, '4, '5, '6, '7, '8 be the path of a particle of ether 
through the atom marked by seventeen points corresponding to 
the same numbers unaccented showing the orbit of the same 
particle of ether on the former supposition. On both suppositions, 
the position of the particle of ether at time 10 from our original 
era (§ 10), is marked 0. For times 11, 12, 13, etc., the positions 
of the particle on the former supposition are marked 1, 2, 3, 4, 5, 
6, 7, 8 on the left half of the orbit. The positions of the same 
particle on the present supposition are found by drawing from the 
points 1, 2, 3, ... 7, 8 parallel lines to the right, 1 '1, 2 '2, 
3 '3, . . . 7 '7, 8 '8, equal respectively to *1, *2, *3, . . . *7, ’8 of the 
radius of the atom, being our unit of length. Thus we have the 
latter half of the passage of the particle through the atom ; 
the first half is equal and similar on the left-hand side of the 
atom. Applying the same process to every one of the ten orbits 
shown in fig. 4, and to the nine orbits of particles whose 
undisturbed distances from the central line on the other side are 
*1, *2, . . . *9, we find the set of stream-lines shown in fig. 5. 
The dots on these lines show the positions of the particles 
at times 0, 1, 2, ... 19, 20 of our original reckoning (§ 10). 
The numbers on the stream-line of the particle whose undisturbed 
distance from the central line is *6 are marked for comparison 
with fig. 3. The lines drawn across the stream-lines on the 
left-hand side of fig. 5, show simultaneous positions of rows of 
particles of ether which, when undisturbed, are in straight 
lines perpendicular to the direction of motion. The quadri- 
laterals thus formed within the left-hapd semicircle show the 
