I. SURVEYING. 607 



screws at the right hand of the instrument are moved simultaneously. The 

 value of one division of these screws corresponds to a change of inclination 

 of the upper bar, of one second of angle. 



2952. Standard Toise. 



This bar of cast steel was formed to be a connecting link between the 

 English yard and the (( toise," which is the geodesic unit of length of so many 

 countries of Europe. Its length in terms of the standard yard (about 76*74 in.) is 

 known, from some 4,000 observations, with a probable error of one ten-millionth 

 part of itself. It has been compared with the standard toise of Belgium, with 

 that of Prussia, with the standard double toises of the Russian Geodetic Survey, 

 with a standard toise of Vienna, and with the Spanish four-metre bar. 



Connecting the survey of so many countries, this standard bar takes a place 

 of the utmost importance in the determination of the figure of the earth. 



The continental toises are generally measures " by contact ; " that is, the 

 measure is not indicated by lines on the bar, but by the entire length of the bar 

 between its extremities. 



2953. Thermometer Calibration Apparatus. 



It is necessarv in the comparisons of standards that the errors of the ther- 

 mometers should be known to two or three hundredths of a degree Fahrenheit. 

 The calibration errors of the best standard thermometers (such as those made 

 by Casella for the Ordnance Survey) are extremely small, but it is necessary 

 to determine what they actually are in order to correct the observations made 

 with them. The apparatus consists of a cast-iron frame, having two parallel 

 rods above, on which slides a microscope. The thermometer lying in a carriage 

 below is read by the microscope, and by means of the micrometer screw on 

 the right a small movement is communicated to the carriage and thermometer. 

 Now, if we break off from the column of mercury a piece, say of 30 in length, 

 and cause that piece to take different positions in the tube, it will be seen that 

 we can by means of the micrometer and the microscope detect errors among 

 the division lines ; and finally determine as many errors as we choose. 



2954. Isometric Drawing of Expansion Apparatus. 



This drawing shows the interior of the room in which the comparisons of 

 bars are made ; it is a room 20 feet by 11, half sunken below the surface of 

 the ground, and entirely surrounded and covered over by an outer building. 

 In the drawing are shown the three stone piers for carrying microscopes. In 

 the experiments depicted, two microscopes are used and stand upon the outer 

 piers. The long boxes contain bars, one hot, the other cold ; they are alter- 

 nately brought under the microscopes. In this manner the absolute expansions 

 of two Indian standards, two English standards, and two American standards 

 have been determined. The drawing shows the flexible tubes which form 

 supply and waste pipes for the currents of water, hot and cold, which maintain 

 the bars at steady fixed temperatures. 



SURVEYING INSTRUMENTS EXHIBITED BY THE ORDNANCE SURVEY 

 DEPARTMENT OP THE ROYAL PRUSSIAN GENERAL STAFF, 

 BERLIN. 



2955. Distance Measuring Plumb-line (ancient con- 

 struction by Breithaupt, No. 71). 



2956. Distance Measuring Plumb-line (construction of 

 1874, by Breithaupt, No. 97). 



