accept Conditions founded upon the reputation of the graduating 

 engine used by the manufacturer, and for ordinary work will read 

 only one vernier. 



If two verniers are not set exactly 180 apart the error of align- 

 ment can be overcome firbt, by using only one vernier, which keeps 

 the error constant and negligible and second, by taking the mean of 

 two \ernier readings at every observation. 



The Stadia 



The invention of this tachymetric principle should be properly 

 attributed to G. Montanari who published at Cologne in 1674 and 

 at Venice in 1680, a method of placing many equidistant filaments 

 of known value on the diaphragm*. 



The Danish Acad. of Sci. awarded a prize to G. F. Brander 

 for the stadia he used with an alidade in 1772, and in 1777 the Soc. 

 of Arts presented an award to William Green of London for the 

 same invention. The great inventor and engineer, James Watts, 

 however, was known to have used the stadia interval as we use it to- 

 day for field work in 1770**. Prof. Fontana of Florence proposed 

 spider lines instead of human hair or silk filaments in 1775. Ramsden 

 was doubless the first to mount spider webs and Rittenhouse of Phila. 

 followed his example in 1786. The first extensive use of the stadia 

 method in the U. S. was organized by J. R. Mayer in connection 

 with the Great Lakes Survey ii< 



When the transit instrument is supplied with equally and 

 accurately spaced stadia*** wires, a telescope bubble, and a verti- 

 cal limb, it is capable of the rapid location of points by the polar 

 coordinates of azimuth, elevation and distance, with reference to 

 some datum, which gives it the right to be designated as a "Tachy- 

 meter". If the telescope possessess sufficient illumination and power 

 the stadia method is even more accurate than the chain, and if the 

 toll-scope bubble is correctly associated with a properly constructed 

 sight-line, the percentage of error in elevation will be even less. 

 Chaining errors are cumulative while the errors of observation in 

 stadia surveying tend to compensate one another****. On this 

 account the stadia has very rapidly superceded all other topographic 

 methods for small scale maps. 



* Gfnmetrfa Applicata. A Salmoraglr ', >. 278. 



** }>r,,f. J. L. Ian Ornum. /tut. of Um !. / P. 3S4. 



*** The word stadia, being the plural of stadium, is of Roman or 



It :t a.\ // V of a A' hf word "j/.;.. 



to dtiiKnate the nut, as originally intruded, f>u( it has ntnv come to signify the 

 mfthoii only. 

 **** Prin. antt Pract . ffotmtr, 1WS. I 'ol //. / 



