“Yet Tycho and Hevelius often erred several minutes in the determination of a star’s place, and it was a great achievement of Roemer and Flamsteed to reduce this error to seconds. Bradley, the modern Hipparchus, carried on the improvement, his errors in right ascension being under one second of time, and those of declination under four seconds of arc according to Bessel. In the present day the average error of a single observation is probably reduced to the half or quarter of what it was in Bradley’s time; and further extreme accuracy is attained by the multiplication of observations, and their skilful combination according to the method of least squares. Some of the more important constants, for instance that of nutation, have been determined within the tenth part of a second of arc. “It would be a matter of great interest to trace out the dependence of this vast progress upon the introduction of new instruments. The astrolabe of Ptolemy, the telescope of Galileo, the pendulum of Galileo and Huygens, the micrometer of Horrocks, and the telescopic sights and micrometer of Gascoyne and Picard, Roemer’s transit instrument, Newton’s and Hadley’s quadrant, Dollond’s achromatic lenses, Harrison’s chronometer, and Ramsden’s dividing engine--such were some of the principal additions to astronomical apparatus. The result is that we now take note of quantities 1/300,000 or 1/400,000 the size of the smallest observable in the time of the Chaldeans.” [Illustration: Compass needle deflected by an electric current borne in a wire.] [Illustration: Compass needle deflected by an electric current borne in a coil.] [Illustration: Suspended coil with D, soft iron core. N, S, magnetic poles.] Electricity Measured.