December 8, 1904J 



NA TURE 



137 



the extreme marks. These wires carry at their extremities 

 scales of invar, having the form represented in Fig. 4, with 

 their edges in the same line as the axis of the wire. This 

 arrangement, somewhat complicated in appearance, is 

 necessary to ensure constancy of length, whatever be the 

 inclination of the scale in a transverse direction. 



^E 



During four years measurements have been made weekly 

 with a great number of wires which have been submitted 

 to different treatment. Owing to the complexity of the 

 subject, more than a hundred thousand comparisons 

 between the wires and the base were necessary to elucidate 

 all the questions relating to the stability of the wires and 

 the precision that they guarantee. After four years, and 

 after the method of treatment of the wires has been gradually 

 modified so as to ensure the greatest possible degree of 

 stability, we can emphatically assert the excellence of the 

 method of measurement by wires constructed of invar. 

 When a wire of the usual diameter of 1-65 mm. is stretched 

 by loads varving from an insignificant weight to that of 

 20 kilograms, the permanent elongation which it undergoes 

 is not measurable ; moreover, it can be rolled as often as 

 desired on a drum (Fig. 5) of sufficient diameter (at least 

 50 cm.), or kept rolled for months without showing on 

 subsequent measure^ment a variation greater than that due 

 to errors of observation. .Several wires which were 

 measured at the bureau were returned after use in the field ; 

 in the beginning, variations in the length of the order i in 

 200,000 were observed in several instances, but recently the 

 constancy of the length has become much more decided. 

 Whilst reserving the results obtained by long trials in severe 

 climates, it may be concluded from the results obtained in 

 the laboratory that a surveying expedition equipped with 

 several wires constructed of invar and subject to mutual 

 control will be able to measure several long bases without 

 fearing a departure from accuracy in the wires greater 

 than that permissible in such measurements, assuming, of 

 course, that the wires are always handled with due care. 



The considerable increase in the accuracy of geodetic 

 measurements, caused by the substitution of wires of invar 

 for those of steel or brass, necessitated a corresponding 

 improvement in the apparatus. We have therefore proposed 

 certain new principles which have been realised in instru- 

 ments constructed with the aid of M. Carpentier, of which 

 a provisional model has been already mentioned in Nature." 

 A description of the final types which have been adopted 

 would carry me too far ; Fig. 6, which indicates one of the 

 measures, may take its place. It will be sufficient to add 

 that, thanks to the new material which has been discovered, 

 the measurement of a base by means of wires answers all 

 1 June 2, 1904, vol. Ixx. p. 104. 

 NO. 1832, VOL. 71] 



the needs of a surveyor ; the relative error of the base has 

 fallen below that of the angles ; bases can be measured 

 across broken ground, cultivated land, streams and rivers. 

 Above all these advantages, the complete staff, including 

 auxiliaries, need not exceed ten men for a rate of progress 

 of 5 kilometres per day. This arrangement, compared with 

 that by which ten years ago fifty men using rules and 

 microscopes could advance 500 metres a day, exhibits an 

 economv of gS per cent. ! To-day the measurement of a 

 base with all the accuracy required in geodesy costs little 

 more than chaining, and the proof has been so thorough 

 that the French Survey finds its advantageous to measure 

 all its bases by the new method. 



The advantages of measurements by wires have been 

 quiiklv recognised by surveyors. Several departments of 

 survey have requested the Bureau international to 

 standardise wires suitable for base measurements; we have 

 thus had the satisfaction of examining the apparatus for 

 use by the Argentine Republic, Australia, Cape Colony, 

 France, Germany, Japan, Mexico, Roumania, Russia, 

 Servia, and Switzerland. 



This simplification in the fundamental measurements of 

 the survey will lead to a reversal in the future of the re- 

 spective positions of the base and angular measurements. 

 In the old method of surveying measurements of bases were 

 reduced as much as possible and angles multiplied inde- 

 finitely ; in the new geodesy angles will be controlled by 

 frequent measurement of numerous long bases. This 

 general plan has already been introduced in the United 

 States in the fine work carried out during the determination 

 of the length of the 98° meridian. 



Horology and Chronometry.— The possibility of con- 

 structing a compensated pendulum with its rod of invar 

 is so obvious that it is hardly necessary to emphasise it. 

 It will be sufficient to observe that the slight change which 

 invar undergoes is not for this purpose a serious defect. 

 As it is necessary to determine the rate of a clock at fre- 

 quent intervals, variations in the daily rate of the order 



Fig. a.— Diagram of the compensation of a chronometer with^a steel-brass 



of a few hundredths of a second in a year will be merged 

 in the variation of the longer period, and will give rise to 

 an error hardly to be feared; but other applications will 

 need some explanation. 



In order not to prolong the preliminary part of this article, 

 I omitted to mention a singular properly of the nickel-steels. 



