. Spons' dictionary of engineering, civil, mechanical, military, and naval; with technical terms in French, German, Italian, and Spanish . r -\-h A2 &c. [3] GEODESY. 1651 nearly. In order, therefore, to reduce the base to the level of the sea, we must subtract the LAcorrection — from the length. We will now give, as an example, the final result of the measurement of the first base, withglass rods, on Hounslow Heath. (Trig. Survey, vol. i., p. 87.) length of the base as measured by 1369 • 925521 glass rods, of 20 ft. each + 4-31 ft 27402-8204 Keduction of the hypothenuses, to
. Spons' dictionary of engineering, civil, mechanical, military, and naval; with technical terms in French, German, Italian, and Spanish . r -\-h A2 &c. [3] GEODESY. 1651 nearly. In order, therefore, to reduce the base to the level of the sea, we must subtract the LAcorrection — from the length. We will now give, as an example, the final result of the measurement of the first base, withglass rods, on Hounslow Heath. (Trig. Survey, vol. i., p. 87.) length of the base as measured by 1369 • 925521 glass rods, of 20 ft. each + 4-31 ft 27402-8204 Keduction of the hypothenuses, to be subtracted -^ 0*0714 Add the difference between the expansion of the glass above, and the contraction of it below, 62° +0-3489 Add also for 6° difference of temperature of the standard brass scale and the glass rods + 0 - 9864 Length of the base, in temperature 62° .. ..■ 27404-0843 Eeduction from the height of the lower end of the base above the mean level of the sea, supposed to be 54 ft., —0-0706 True length of the base, reduced to the mean level of the sea 27404-0137 ,>D The Measurement and Reduction of the Angles.—In all the surveys made in the British dominions,the instrument for measuring angles has been a large theodolite, rendered as perfect as theingenuity of English artists could make it. This instrument may be defined to be an altitudeand azimuth instrument, or an instrument for measuring vertical and horizontal angles. Thehorizontal circle was 3 ft. in diameter, and angles could be measured-with it to the fractional partof a second. The instrument used by the French and Swedish surveyors was the repeating circle of principle of the circle of repetition is to take the angle several times successively in continua-tion on the circle, and then divide the whole arc by the number of observations. Let A B D, Fig. 3225, be a circle graduated entirely round the circumference from right to lefton the upper side only of the instrument. A a,
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