or THK UMTEU STATES. 4*88 



The local point of tlie cobwcl) on the bar being dftcrmincd 

 with srcat accuiacy jn lespcct to its elevation, (as is evident 

 iVoni tlie al)sence ol" parallax between the two olyccts,) we 

 may measure from it in any inclination that occurs, without 

 ftar of error in the distance. 



The following is the arrangement added to the i)ar appa- 

 ratus to measure this inclination. It is seen in both figures 

 of Plate IV. hut more ])articularly in Plate 111. fig. 4, 5, fj R. 



Upon the strong brass frame K. forming the leading part 

 of the wooden bar H, a sector of ten inrlics ladius is screwed 

 by its strong edge bar mtn, (fig. 4) and presents itself verii- 

 tically, its centre being at A, and the divided arc ff, at the side 

 of the head I of the screw Q. It contains an arc of upwards 

 of thirty degrees. Upon the alhidadc of this sector there 

 is a spirit level of al)out seven inches long, ad)ustal)lc to the 

 0'' of the sector by the screw i, which makes it revolve about 

 the point /r. The alhidadc being moved by the tangent 

 screw racking in the circumfeiencc of the arc c^. until the 

 level be obtained with the alhidade corresponding to any in- 

 clination of the apparatus, the reading of the vernier on the 

 arc will indicate the inclination of the ai)paratus or the bars 

 to whicli tlie sector has been adjusted. 



For the reduction of the inclined distances to the horizon- 

 tals, it is evident that a table of differences between the hy- 

 potenuse and base of a right angled triangle can be con- 

 structed to any degree of accuracy desired, the quantities of 

 which referring always to the same unit, viz. the length of 

 one full system of bars, will be taken out without any calcu- 

 lation, and their sum can be subtracted from the sum total at 

 once. 



Respectitig the thermometers, it will not be necessary to 

 say any tiling here, as it is easily conceived that they must 

 all be read at each laying of a box. 



1 might now (lescril)e the manipidation of this apparatus 

 in the actual measurement of a base line, but I consider the 

 use of each part of it so obvious to a person sufficiently ac- 



