The reburnt and cooled char is collected and sent back to the char cisterns.
On the north-east the principal heights are Ben Slioch (3217 ft.), whose sugar-loaf form dominates the landscape, Ben Lair (2817) and Ben Airidh-a-Char (2593), and, on the south-west, the peaks of Ben Eay, four of which exceed 3000 ft.
Below each retort, and attached to it, is a cooler formed of thin sheet-iron, which receives the hot char as it passes from the retort, and at the bottom of the cooler is an arrangement of valves which permits a certain amount of char to drop out and no more.
In the best-appointed refineries the whole of the work in connexion with the char is performed mechanically, with the exception of packing the filter cisterns with fresh char and emptying the spent and washed char on to the carrying bands.
Differences not only in the size of towns, but in the arrangement and char - acter of the population, make each district a telephone problem b3, itself, and nullify close comparisons between telephone rates and telephone efficiencies in different areas and different countries.