THEODOLITE. 



pofition ; ai\d in both fUuitions the telcfcope is capable of 

 Itavlng d circuloi- motion, tliat carriss the attached level 

 round \<ith it an entire revolution. During this revolution, 

 an eye obferving the interfeiled point of the field of viev?, 

 as projeded on a diilant point, will fee whether or not aay 

 adjultment of the fpider's lines is neceffary, and in what 

 relpeft. The aperture of each telefcope is an inch and a half, 

 and the ordinary magnifying powers are from teu to eleven, 

 with nearly fourteen inches of tute ; but the upper or raca- 

 furing telefcope has a fecond eye-piece of the pofitive kind, 

 whicii produces a magnifying power of twenty times. There 

 are tliree fpider's lines in each eye-piece, one horizontal, and 

 two crolTnig it fo as to include a fmall angle between 

 them ; which method of fixing the lines, allows the obferver 

 an opportimity of bifecting the faid fmall angle by a ver- 

 tical ilaff ereftcd at a dillance, which is better than cover- 

 ing the ftafT with a vertical line, that would dilappear upon 

 the pknc of fuch ilaff. Before we proceed to explain how 

 this tJieodolite is ufed in the field, it will be requifite to 

 fliew how the pre\-ious adjuftmcnts are to be performed. 



When the iiiltrument is fcrewed to the head of the tripod, 

 the legs mull be opened wide enough to enfure a firm pofi- 

 tion, and the points muil be preffed into the ground equally, 

 cxaAly over the hole into which a ftation ftafF has been, or is 

 intended to be, inferted ; fo that the plumb, fufpended from 

 a pin at the junftion of the legs, will fall exadlly upon the 

 faid hole ; in which fituation the Jiaiion, marked 0, is faid 

 to be taken. 



When the theodolite has been properly fixed in its ftation, 

 the firft adjuftment that will require to be made, is that 

 wliich regards tiic line of collimaiion. When all the parts of 

 the inftrument are properly in their places, let the upper 

 telefcope be pointed in a horizontal line, that paffes over 

 two of the four fcrews of plate D E, and note what point 

 in a diftant objedt is covered by the horizontal fpider's line, 

 near the middle of the field of view ; then turn the telefcope 

 half round in the Y's, till the level lies above it, and obferve 

 if the fame point is again cut by the faid line ; if not, ele- 

 vate or lower the horizontal line by the proper fcrews in the 

 eye-piece, releafing one and fcrcwing up the other, till the 

 diftance between the two points, correfponding to the two 

 pofitions, is bifedled by the horizontal fpider's hne : and if, 

 vrken the telefcope is turned back to its original pofition, 

 with the level under it, the fpider's line covers the point ad- 

 jufted to, the line of coUimation in altitude or depreffion 

 will be correft ; but if not, the operation muft be repeated 

 delicately, till the horizontal line covers the fame diftant 

 point in both pofitions. The fame operation will alfo put 

 the vertical hne correft, or rather the point of interfedlion, 

 when there are two oblique lines inftead of a vertical line, 

 as in Troughton's theodolite ; and the adjuftments will be 

 known to be complete, when the point of interfedlion con- 

 tinues on the fame diftant point, while the telefcope is made 

 to revolve round the line of colhmation as an axis. 



The fecond adjuftment is that which puts the long level pa- 

 rallel to the rcftified line of collimation. While the telefcope 

 remains parallel to the line that joins two oppofite fcrews 

 of plate D E, adjuft thofe two fcrews by turning them in 

 oppofite direftions, until the bubble is obferved to be in the 

 middle of its tube under the telefcope : then, the femi-rings 

 V and W being previoufly turned back, reverfe the ends of 

 the telefcope, and alfo of the level attached to it ; and if the 

 bubble will refume its former fituation in the middle of the 

 tube, both the line of collimation and the level will be truly 

 horizontal, and confequently parallel to each other ; but if 

 the bubble recedes to one end, bring it back one half by the 

 fcrews that elevate or deprefs one end of the level, and the 



10 



other half by the fcrews of plate D E. Let this operation 

 be repeated till the bubble will remain in tlie middle of the 

 tube, after the telefcope has been reverfed into both pofi- 

 tions. The adjuftment of tJie level, however, is not yet com- 

 plete ; for though the axes of the two tubes may be equi- 

 diftant at both ends of the level, yet may they be in- 

 clined fo as to form an horizontal angle with each other 

 at fome diftance, and in this cafe the bubble will run to 

 the higher end of its tube, when the telefcope rotates ; 

 there are therefore two lateral fcrews, which adjuft the paral- 

 lelifm of the two tubes, fo that the bubble will remain in 

 the middle, in every part of a rotation of the telefcope 

 round its line of collimation, which adjuftment muft now 

 be made ; and if this lateral adjuftment ftiould be found te 

 derange the vertical one before made, it muft be re-adjufted, 

 and the level will then be in a proper ftate for ufe, provided 

 the Y's are alike, and alfo the cylinders that reft in them ; 

 but if not, the bubble wfll not reft in the middle in both of 

 the reverfed pofitions, tiU they are made fo. 



The third adjuftment that we propofe to explain, is that 

 which puts the line of collimation exaftly at right angles 

 to the axis of the femi-circle's vertical motion, which ha."; 

 not yet been defcribed, but which is eflentially neceftaiy 

 to be attended to, more particularly by the maker. Let 

 the upper telefcope be diredled to fome horizontal well- 

 defined mark, that is included within the angular fpace 

 formed by the two obhqiie lines, juft where the vertical 

 line would have been, if fuch line had been ufed, while zero 

 of the vernier coincides with zero (360*) of the horizon- 

 tal limb of plate L M ; then all the fcrews being faft, ex- 

 cept the one which allows the vernier-plate to revolve, turn 

 180° in azimuth, and reverfe the telefcope by taking it out 

 of its Y's, into its original pofition with refpedl to the 

 mark ; and if this is feen in precifely the fame fituation in 

 the angular Ipace, (though not perhaps with refpedl to alti- 

 tude, if the axis of the vernier-plate was not previoufly 

 adjufted,) the line of collimation will be at right angles 

 to the axis of the vertical motion ; but fliould this not 

 prove to be the cafe, one of the Y's will require to be al- 

 tered laterally, but the adjuftment of the level will not be 

 deranged thereby. This alteration of one of the Y's had, 

 however, better be eff"efted by a regular workman, and 

 ftiould never be negleifled by the maker. 



The fourth adjuftment, which might have preceded the 

 third without inconvenience, is that which makes the com- 

 mon axis of motion of the vernier-plate and horizontal gra- 

 duated circle truly vertical. This is done partly by the 

 fcrews that fix the ball and focket, inferted into plate D E, 

 and partly by the adjufting-fcrew R of the vertical ferai- 

 circle. Hitherto the upper telefcope has lain over two of 

 the four fcrews only, and in a horizontal line, with the bub- 

 ble in the middle. Let it now be turned along with the 

 vernier-plate juft 90°, till it lies over the other pair of op- 

 pofite fcrews, and fee if the bubble be now alfo in the 

 middle, if not, make it fo by thofe fcrews ; and if, after this 

 operation, the bubble will remain in the middle during a 

 whole revolution of the vernier-plate, in both the reverfed 

 pofitions of the telefcope, the vertical axis of motion will 

 be truly adjufted. 



The fifth adjuftment relates to the levels fixed to the 

 plane of the horizontal graduated circle, which are ufe- 

 ful in watching the horizontal pofition of the inftrument, 

 while an obfervation is making ; thefe, which are at right 

 angles to each other horizontally, muft alfo be feparately 

 at right angles to the vertical axis that carries them round. 

 When this axis has been adjufted, as we have juft explained, 

 by the level of the upper telefciape, the bubbles of both 



the 



