June 1, 1897.] 



KNOWLEDGE. 



149 



of the good people of Hammerfest. The column and 

 pedestal are of granite, and at the top is a bronze capital 

 carrying a terrestrial globe of copper. Inscribed on the 



"' The Meridian Column, Ilammerfest. (From a Photograph bv! 

 W. Shackleton.) 



column, in Latin and Norwegian, is the following legend : — 

 " The northern termination of the are of meridian of 25° 20' 

 from the Arctic Ocean to the Hirer Danube, throiifjli Nonvaij, 

 Sweden, and Russia, uhich, according to the orders of His 

 2Iajesti/ King Oscar I., and the Emperors Alexander I. and 

 Nicholas I., and by uninterruiited labours from 1816 to 1852, 

 was measured by the r/eometers of the three nations." 



The aim of the monument is therefore to commemorate 

 the completion of a gigantic piece of surveying, having for 

 its primary object the measurement of the earth, and to 

 provide a permanent mark in order that the measurements 

 may be repeated at any future time if considered desirable. 

 No longer arc of meridian has yet been measured, and it 

 may not be without interest to recall some of the facts 

 connected with this great undertaking, culled mainly from 

 the monumental report which was drawn up by its chief 

 organizer, the celebrated F. G. W. Struve." 



First, however, a few words as to the general principles 

 involved in the operations of ye„desy, or earth measure- 

 ment. 



When once the idea of a globular earth was accepted, 

 the determination of its dimensions became theoretically 

 a very simple matter, involving only the measurement of 

 the length of a portion of a great circle, the number of 

 degrees in which could also be ascertained. One of the 

 arcs which can be most readily measured in degrees is one 

 which forms part of a great circle passing through the 

 Poles, for the reason that the measurements of the 

 difference of latitudes of the terminal points furnishes the 

 requisite angular value. In this way the measurement of 

 the earth is usually carried on by measuring so-called 

 arcs of meridian, from which the length of a degree, and 

 therefore the circumference and diameter of the earth, at 

 once become known. If the earth were really spherical, 

 the measurement of even a single degree would suffice for 



* "Arc du lleridien de 25" 20'entre 1 • Danube et la Mer G-laciale." 

 Par F. G. W. Strure. (St. Peterabourg, 1862.) 



the determination of its dimensions ; but, as degrees are 

 found to be of different lengths, it becomes necessary to 

 measure a great number of them, in various parts of the 

 world, in order that the precise size and shape may be 

 investigated by analytical geometry. 



Two distinct operations are requisite. The first has for 

 its object the accurate measurement, in linear measure, of 

 the length of the meridional arc ; the second the astrono- 

 mical determination of its angular value on the globe. 



The first of these operations depends primarily upon the 

 measurement of a base line, some miles in length, upon 

 the most level stretch of land available. The laying down 

 of a base line is a most tedious and delicate process, for 

 even if it be seven or eight miles long, it must be as straight 

 as possible, and its length must be ascertained almost to the 

 nearest inch. One of the greatest difficulties to be faced is 

 the fluctuation in the length of the measuring rods, con- 

 sequent upon changes of atmospheric temperature, but this 

 has been ingeniously overcome by the use of the compen- 

 sating bars invented by Lieut. Colby. A bar of brass is firmly 

 joined at its centre to one of iron, and pivoted at the ends 

 are a pair of projecting metal " tongues," certain marks on 

 which remain at a constant distance, because the expansion 

 of one rod pushes them apart just as much as that of the 

 other endeavours to bring them nearer together. Such 

 rods cannot be put in contact end to end, and they are 

 accordingly placed a constant and known distance apart 

 by means of a pair of microscopes, which are joined 

 together by compensation bars. Sights and levels to 

 ensure the straightness of the line complete the essential 

 parts of the apparatus. 



The base line having been laid down, very accurate 

 theodolites are placed at its extreme ends, and a distant 

 object which can be seen with both instruments is selected 

 for observation. At each end the angle between the base 

 line and the object is measured, and the distance of the 

 object from either end then follows from a simple trigono- 

 metrical calculation. The sides of such a triangle may 

 now serve the same purpose as the original base line, and 

 other objects, which are often specially erected signals, are 

 similarly sighted and their distances measured. 



By this system of trianyulation, the measurement is 

 carried on to any desired extent, being checked here and 

 there by the actual laying down of new base lines, the 

 lengths of which have been previously determined in the 

 course of the trigonometrical surveys. The general dis- 

 position of successive triangles will be gathered from the 

 accompanying map, representing the actual triangles 

 measured near the southern end of the great Eussian arc. 

 From the calculated dimensions of the individual triangles, 

 the distance between the extreme ends of the arc can 

 readily be deduced. 



For the determinations of latitude of the extreme or 

 intermediate points, meridian circles, zenith sectors, or 

 other suitable instruments of high precision are employed, 

 so that finally both linear and angular measurements of 

 the same arcs become available for the calculation of the 

 earth's dimensions. 



The oldest triangulation of which history tells us was 

 made by Snellius, in Holland, before 1G17 ; but work of 

 precision may be said to have begun when Picard, in 1666, 

 replaced the sights previously employed with surveying 

 instruments by a telescope fitted with cross wires. This 

 new feature not only introduced greater accuracy, but 

 greatly enlarged the possible extent of the triangles. 

 Picard himself measured a small arc, and deduced the first 

 value of the earth's radius which approached precision. 

 Prior to this, the English mile, which professed to state 

 the length of a minute of arc, was one-sixth too great, and 



