Sept. 13, 1883J 



NATURE 



465 



aimed at by these arrangements is to secure to the utmost 

 degree possible the continuance of a uniform temperature 

 which in certain operations is a condition of success. 



The labours of the bureau are naturally divided into two 

 sections ; one dealing with standards of length, the other 

 with standards of mass or weight. The first comprises 

 principally the settlement of equations of the various 

 standards, that is of their lengths compared with the 

 prototype which forms the universal basis of departure ; 

 the ratio of their expansions ; the study of their sub- 

 divisions or of their multiples ; and particularly of the 

 great rules (figles) which serve geodesistsforthe measure- 

 ment of terrestrial bases. Similarly, the sec- 

 tion of weights is busied with determining the 

 relations of several primary kilogrammes to 

 the kilogramme prototype, with the adjust- 

 ments of their subdivisions, with the com- 

 putation of their specific weights, &c. These 

 various labours are divided among a certain 

 number of experts constituting the scientific 

 staff of the bureau. We shall hastily indicate 

 the principal instruments used in both sec- 

 tions, instruments which are the workmanship 

 of the ablest mechanists in the whole of 

 Kurope, and which in general show the last 

 limit of perfection attainable in this precise 

 branch of mechanics. 



The instruments belonging to the section 

 of linear standards or metres, are called 

 " comparateurs.'' A comparateur for metres 

 d traits is essentially composed of two micro- 

 scopes solidly fixed and immovable, provided 

 with micrometers under which, by a peculiar 

 mechanism, the two rules it is desired to com- 

 pare with each other can be successively in- 

 troduced. The bureau possesses several of 

 these instruments, each adapted to a special 

 purpose, and consequently distinguished by 

 peculiarities of construction. 



The first is Brunner's comparateur, de- 

 signed for comparisons in air of metres a 

 traits. The two microscopes are by means 

 of cantilevers riveted on large stone pillars 

 or monoliths resting on massive masonry. 

 The micrometers with which they are fur- 

 nished follow the arrangements usually ob- 

 served in the case of astronomical instru- 

 ments. Each is composed of a kind of 

 little rectangular box, lengthened and flat- 

 tened, fixed on the body of the microscope 

 below the eyepiece. In this box is a frame 

 capable of being displaced from right to left. 

 On the frame are stretched two spider threads, 

 very fine, parallel and very close to each other, 

 which constitute the parallel spider threads 

 or reticule. The displacement of the frame 

 is effected very slowly by means of a micro- 

 meter screw of perfect workmanship, which 

 is worked from the outside by means of an 

 enlarged micrometer-head or drum, the cir- 

 cumference of which is divided into a hundred 

 tqual parts. By turning this round the experi- 

 menter moves the screw, which in turn moves the frame and 

 displaces the spider threads visible in the field of the micro- 

 scope. The image of the lines traced on the rule as given 

 by the objective then lies in the plane of these threads. 

 To bisect a line is to make the parallel threads coincide 

 with the image of this line, that is by turning round the 

 drum so as to bring the threads into such a position that 

 the line may appear exactly in the middle of them ; the 

 position occupied by the parallel threads will in that case 

 be indicated by the reading of the micrometer-head or 

 drum. Should a second line appear under this microscope 

 in a different position, it will be necessary, in order to 



bisect it in turn, to displace the threads, that is to make 

 the drum revolve a certain number of divisions. The 

 distance corresponding with one displacement of a division 

 being known, the distance between the first and second 

 lines can then be calculated. 



Under the microscopes is the body itself of the com 

 parateur, composed primarily of a strong framework of 

 brass, exceedingly massive and steaiy, forming on its 

 upper borders a kind of railway on which rolls a heavy 

 carriage, movable at pleasure by a handle which controls 

 a system of cog-wheels. Surmounting this carriage is a 

 long box or metal trough with double walls formed of two 



Fie. 2. — Balance of Precision for Comparison of Standard Kilogrammes. 



cases inclosed one within the other. This box is designed 

 to receive the two rules that are to be compared. These 

 are placed beside each other in the middle of the box on 

 supports of suitable form. The box is provided with 

 various contrivances by means of which the observer, 

 while observing the microscopes, is able to adjust the 

 rules ; to lower them or to raise them ; to bring them 

 into focus at the two extremities, or to displace them 

 longitudinally or transversely, as may be required. The 

 box is able, moreover, to receive a sufficient number of 

 thermometers, which are observed with the aid of special 

 lenses fitted in the lid which covers the whole and which 



