brass at a certain temperature. Now, many bases have been measured with, bars or chains of iron or brass, but in every part of the operation every possible care has been used to screen them from changes of temperature, by covering them with tents; putting perhaps half-a-dozen bars at a time in a row, with twenty yards of tent over them, so as to protect them effectually from the sun and wind. Having taken these precautions to guard them from the effects of changes of temperature, thermometers are placed by the side of the bars. Then by carefully observing the state of the thermometer, and knowing the expansion of the bars by heat, or their contraction by cold, we can ascertain what length these bars represent under the circumstances under which they are used. But there was another contrivance used specially in the Loch Foyle base. It was used for the first time there; it has been since used in India, and at the Cape of Good Hope. Figure 16 represents
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Fig, 16.
a combination of two bars; one, ABC, of brass, and the other, DEF, of iron, connected at the middle BE, and having projecting tongues, ADG and CFH, which are connected with both bars at the two ends of the bars. Now, the use of the combination is this: brass expands by increase of warmth considerably more than iron, in the proportion of 5 to 3, as nearly as possible. If I arrange the bars in this manner, and choose points G and H in such positions that
