Page:Popular Astronomy - Airy - 1881.djvu/63

Rh DG is three-fifths of AG, and FH is three-fifths of CH; then the distance between H and G is not disturbed by the expansion of the two bars. The iron bar expands, the brass bar expands more; and by that increased expansion of the brass bar, the two points G and H are brought inwards by exactly the proper quantity. In this manner a means of measuring has been attained, which, in the judgment of many persons who have used it, is better suited to the purpose than anything else that has been used or adopted. I have described in detail this apparatus to shew the extreme caution necessary in these matters.

A succession of combined bars like these are placed one after another, with a small interval between each and its successor; and then the question is, how is the interval between them to be measured? It will not do to make one bar touch the other, because expansions may be going on in one of the series of bars, and it would jostle the others throughout the whole extent. In the measure of which I spoke, this small distance was measured by means of microscopes; and these microscopes were so mounted (on the same principle as the bars) that the measure which they gave was not affected by temperature. In some of the surveys on the Continent, glass wedges have been dropped between the successive bars; in some others, there have been sliding tongues used; indeed an infinity of contrivances have been used to overcome the difficulty. The effect of all this has been, that a distance of 8 or 10 miles has been measured to within a very small fraction of an inch. This is the first application of the yard measure, by which the distances of the sun, the moon, and the stars are to be measured. In figure 17, EF represents the base 4em