44 SEC. 3. MEASUREMENT. 



The bevel edged set square side is used to show the divisions coinciding ; 

 and the equivalent values of English and Foreign measures of length may thus 

 be readily obtained. 



245. Plotting Scales. Ivory. Two specimens, to show fine 

 and accurate dividing. Aston $ Mander. 



No. 1 shows two chains to the inch, represented by 200 divisions to the 

 inch. 



No. 2 shows one chain to the inch, represented by 100 divisions to the 

 inch. 



247. Longitudinal Measure, according to natural princi- 

 ples. Hans Baumgartner, Basle. 



250. Half Metre, maple, with points. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



251. Brass Metre. (Model of the Grand Duchy of Baden.) 

 Geneva Association for Constructing Scientific Instru- 

 ments. 



252. Double Steel Metre, with points. (German Model.) 

 Geneva Association for Constructing Scientific Instru- 

 ments. 



253. Brass Standard Metre. (Swiss Model.) 



Geneva Association for Constructing Scientific Instru- 

 ments. 



The Geneva Association for Constructing Scientific Instruments possesses 

 in its laboratories a machine for the division of straight lines, to the con- 

 struction of which it has endeavoured to apply all the improvements of 

 modern science. Its efforts have been crowned with success, and now the 

 increasing reputation of this machine, which may be considered as the most 

 complete at present existing, has gained for the Geneva Association orders 

 for metrical standards from several European Governments. 



The machine is worked automatically, that is, all the process of dividing is 

 done mechanically. Thus it avoids, apart from the inaccuracy consequent on 

 the temperature of the operator, the errors proceeding from the inattention or 

 fatigue of the latter. Mechanical action has, moreover, the advantage of 

 being more regular, seeing that the motive power is always equal. 



An ingenious contrivance enables the correcting, during the division of 

 errors due to a change of outward temperature, and to effect at any tempera- 

 ture an exact division at 0. By the same means, a division of any length 

 may be made, however immeasurable may be ratio of the thread of the screw 

 of the machine to the length of the division required. 



The curve of the screw has been thoroughly studied and corrected, so as to 

 guarantee accuracy to the -^ of a millimetre. 



This machine for dividing straight lines has been used to effect the normal 

 division of the large machine for dividing circles which stands by its side on 

 the same bed of concrete. This application has been the means of exactly 

 ascertaining the coefficient of dilatation of the machine for dividing straight 

 lines. The maximum of error found in the division of the normal circle was 

 less than a second. It is impossible to expect greater accuracy when it is 

 remembered that the arc of a second on the circumference of the divided 

 circle represents about -^^ of the millimetre. 



