952

THEORY AND PRACTICE OF ENGINEERING.

Book II.

accurately noted when she changes her course; then, proceeding in the same manner asalready described, the soundings and remarks will enable a tolerably correct sketch of thecoast to be made. Distances are sometimes measured by sound, which travels at the rateof 1090 feet per second of time; a chronometer will enable a distance of several milesto be determined with considerable accuracy : the time between the flash and the reportof a gun being found, the distance may be calculated by the above rule.

Trigonometrical Surveying .— The survey of a country, when carried on upon an extensivescale, has much of the labour of admeasurement abridged, by merely applying the rods orchains to a base line, and calculating the other distances by triangles ; the difficulties whichattend the operations of a trigonometrical survey of a country must not, however, beunderrated, particularly when the object is to determine the distances as well as the positionsof places with the greatest precision. Upwards of one million sterling had been granted inthe year 1841 by the Houses of Parliament at several times to carry out the Ordnancesurveys of the Urtited Kingdom. A general survey seems to have been commenced inthe year 1783, in consequence of an application made by the French government toMr. Fox, then Secretary of State of the Foreign Department, to carry a chain of trianglesfrom London to Dover, which could be connected with those of the French arc of themeridian already extended to Dunkirk; so that by actual measurement the relativepositions of the observatories at Paris and Greenwich might he determined. This ap-plication was laid before the Royal Society by Cassini de Thiery, and the great advan-tages which would result from it were highly appreciated by that body. The English government, in consequence, employed General Roy for the operation, who had alreadycommenced a survey of the neighbourhood of London , for the express purpose of connectingthe private observatories with that of Greenwich: as early as 1747, a survey of the High-lands of Scotland was undertaken by this officer, which was considerably advanced by theend of the year 1755. To obtain an accurate base line, so that by its measurement allthe other triangles constructed on it should be computed, was the first part of the opera-tion : and for this purpose General Roy selected a line on Hounslow Heath, in consequenceof the level surface the ground presented, and its proximity to the Royal Observatory atGreenwich . The terminal points of this base line were marked by wooden pipes sunk inthe ground, and the measurement was commenced in June, 1784.

The first mode was with deal rods, which had been employed previously in other countries:but it was soon evident that from the alternations of dryness and humidity, they were subjectto sudden and irregular changes, and by no means fit for a purpose where such greatprecision was necessary. These rods were cut out of an old mast of Riga timber, andmade 20 feet 3 inches in length, tipped with bell-metal, to prevent their ends from beinginjured ; they were 2 inches in depth, and 1| inch broad, and rendered inflexible to a cer-tain degree by trussing; when, however, applied to the standard during the measurement,they were found each to have increased the fifteenth part of an inch, and were conse-quently laid aside : glass rods were then substituted, as tubes of this metal could be easilyprocured of the length required. Three hollow' tubes, 20 feet in length and 1 inch in dia-meter, perfectly straight, were provided, and converted into measuring rods by Ramsden ;they were placed in cases made fast in the middle, and braced at several other points, toprevent their shaking or bending, but not their free expansion and contraction ; the ends ofthese rods were smoothly ground, at right angles to the axis of the tube; one end had afixed metal button attached to it, for making the contacts, and the other a movable sliderwhich, could be pressed outwards bv a slender spring, and against which the fixed extremityof the succeeding rod was placed, and then pressed until a fine line on the slider w'asbrought into exact coincidence with another fine line on the glass rod, when the distancebetween the two extremities was exactly 20 feet. To ascertain precisely the expansionto which these glass rods were subject, they were submitted to a microscopic pyro-meter, when their expansion could he ascertained for every degree of temperature from 32°to 212° of Fahrenheit’s thermometer. „

After much calculation upon the expansion and contraction of the rods they wereapplied to the base line, which was not over perfectly level ground; consequently itbecame necessary to divide the whole distance into inclined lines, each containing about600 feet. This was done by placing the rods exactly in straight lines, stretching from oneextremity of the hypotenuse to the other, and then determining the relative heights ofthe two extremities by means of the spirit level, for the purpose of reducing the horizon ;the cases containing the glass rods being throughout supported upon trestles, which wereabout 30 inches in height. After the measurement was completed, its true length wasreduced to that of the level of the sea, the mean semi-diameter of the earth being estimatedat 3,492,915 fathoms : infinite pains were evidently taken throughout the whole operation,and the means employed seem to have been the best that could be suggested.

The true length of the base was fixed at 27404*0137 feet, hut when the Ordnancesurvey was commenced in 1791, it was thought necessary to remeasure this line, from anidea that as the ends of the two consecutive rods, having been made to rest on the sametrestle, w r hen one was removed the trestle would have a tendency to incline forward, which