we shall say it is on the first ladder; but its situation is now only half determined. These ladders are divided into steps; and each has nine steps. This will be effected by drawing 8 horizontal lines across those already made. (See Plate I. fig. 5.) The point is still on the first ladder, but on what step? It is on the sixth step.
Two rooms should be provided, with four walls in each; in the upper room is to be placed the northern hemisphere, and the southern is to be supposed under the northern, in the lower room. We will begin with the northern hemisphere, dividing it into four equal parts or quarters; transferring one quarter to the first wall. (See Plate I. fig. 6.
The former division of a wall was thus:
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It will not, however, be more difficult to remember nine figures in one line, than in three. The floor of the upper room in which we stand is the equator; upon this we reckon the longitude. From the pole to the equator there are 90°, but we only place 60° on the wall, the remaining 30° being fixed in the ceiling. In each of these squares, there are 10° of longitude, and 10° of latitude.
