1901 - 2 .] Lord Kelvin on Molecular Dynamics of a Crystal. 207 
next-nearest, etc. Fig. 2 shows an equilateral triangle of three 
nearest neighbours, and concentric circles of neighbours in the 
same plane round it. The circles corresponding to r 4 and r 8 of 
§ 7 below, are not drawn in fig. 2. In all that follows the side of 
each of the equilateral triangles is denoted by A. 
§ 6. All the neighbours in assemblage I. are found by aid of the 
Fig. 1. 
(a) The atoms of the net shown in fig 1. The plane of this- 
net we shall call our “middle plane.” Let lines be drawn per- 
pendicular to it through the atom A, and the points marked.^, c, 
to guide the placing of nets of atoms in parallel planes on its two 
sides. 
(b) Two nets of atoms at equal distances A^/-! on ^he ^ wo sides 
of the “ middle plane.” These nets are so placed that an atom of 
one of them, say the near one as we look at the diagram, is in the 
guide line b ; and an atom of the far one is in the guide line c. 
