324 



SECTION-MECHANICS. 



Figure 8. 



engineer has to meet in a particular way. One of these is the case of a 



narrow sound. Suppose, for instance, it were required, as in the case of 



a place in Scotland, to light up a narrow sound. (Shown in figs. 7 & 8.) 



It is quite obvious that if you make the light sufficiently powerful 



to show across the sound, which is perhaps two or three miles 



wide, it would be very insufficient for showing up the sound, where the 



mariner had to see it at seven miles, and still more insufficient for 



showing down the sound where the light is required to be seen for 



fifteen miles. What is required, then, is a system by which the whole 



light from the lamp is spread horizontally with strict equality over any 



given arc in azimuth. Or in a light of equal range, where it must be seen 



at different distances in different azimuths, the light must be allocated 



to each of such arcs in the compound ratio of the number of degrees. 



and the distance from which it is required to be seen. I might 



describe this by a very simple case, but I prefer to allude to the 



case of Buddon Ness condensing light, the apparatus of which may 



