288 
towards precision. The zenith sectors, the mural circles, 
the transits, which, forty years ago, were regarded as almost 
miracles of perfection, are fast sinking into neglect, and are 
displaced by inventions of higher promise. - But instruments 
may be improved in vain unless they be used with skill pro- 
portioned to their improvement, and withdrawn from distur- 
bances which may overwhelm their augmented power. Such 
disturbances, it is my purpose to show, may easily exist in the 
vicinity of a railroad. This matter has already attracted atten- 
tion in consequence of two attempts to carry lines at about 
800 feet from the Observatory of Greenwich, and experiments 
relating to it have been made by Captain Denham, Mr. Airy, 
and myself, whose results, differing only in degree, have been 
published by order of the House of Commons. Captain Den- 
ham, observing at Liverpool, on Sandstone Rock, took alti- 
tudes of the sun with a sextant, of low magnifying power, and 
a small reflecting surface of mercury, and found that the 
image was disturbed to the distance of 1110 feet. Immediately 
above the Tunnel he found the vibration scarcely sensible. 
Mr. Airy’s experiments were made at Greenwich, on gravel, 
and at Kensal Green on clay; viewing with.a small telescope 
the wires of a collimator reflected in mercury, his limits were 
in one case 1100 feet, in the other, 2200. My trials were made 
in the (now vanished) Dodder bank Distillery, 1055 feet from 
the Dublin and Kingstown line, observing with a large repeat- 
ing circle the image of a land object reflected in a circular ves- 
sel of mercury, eight inches diameter. The soil here is mere 
silt, of great depth and uniform density, and, therefore, well 
adapted to propagate tremors. Accordingly, they were ob- 
served as far as the Terminus on one hand, 6434 feet along 
the line, and the Booterstown Station, on the other, 10893 
feet. The wave of earth-vibration was much more extended 
before the engine than behind it, as, indeed, might be expected 
