THEODOLITE. 



tie fixed levels muft be brought to the middle of their re- 

 ipcdivo tubes by their proper fcrews, fo that tlie bubbles 

 ol' all tlie three levels may remain ftationary in every part 

 of tlie revolution of die vernier-plate, which they will do 

 if its axis is truly vertical. 



The fixth adjullment'is that which puts the axis of the 

 femi-circle's motion truly horizontal, or at right angles to 

 the vertical axis of the horizontal circles : this is known to 

 be fo, when the point of interfeftion of the fpider's lines 

 will cover a long plumb-line, fufpendcd at a diftancc, as the 

 angle of elevation increafes, from the inferior to tiic fuperiof 

 end of the fufpended line ; and ftill better by trying if the 

 pole ftar, and its reflefted image, feen in a bafon of quick- 

 filver, will be fuccefTiTely covered by the faid point of interfec- 

 tion of the fpider's lines. In fome theodolites, the horizontal 

 axis lies in adjuftable Y's, like thofe of a traiifit-inltrnment ; 

 but in the inllrument before us, the adjuftment was made 

 permanent by the maker. 



Lailly, the adjuftment of the vernier to zero of the femi- 

 circle is made by the fcrew near M, which elevates or de- 

 preffes the whole frame in which the telefcope and femi- 

 circle move, and confequently alters the fituation of the 

 level, which has its bubble brought back again by the fcrew 

 R, that gives flow motion to the feini-circle, when the 

 clamp at the fuperior end of the arm is made faft to the 

 axis of motion ; but it is not eifential that zero of the 

 vernier (lioiild be at zero of the femi-circle when the level is 

 right, bccaufe when an altitude or depreflion is repeated in 

 the reverfed pofitions of the telefcope, the two i-eadings 

 will have equal and oppofite errors ; and one half of the 

 difference of thofe readings will be the conftant index error, 

 which may be afcertained with great prccifion from an 

 average of a number of reverfed obfervations. 



The lower telefcope, being placed as a guard to watch 

 any azimuthal alteration in the pofition of the inftrument, 

 requires no adjuftments, except that- for diftinft vifion, after 

 it is brought to its objeft by its proper vertical and hori- 

 zontal motions, in which fituation its horizontal motion is 

 clamped by tlie fcrew I under the horizontal plate. 



After the adjuftments are all made, or examined, the 

 theodolite may be ufed for meafuring either horizontal or 

 vertical angles in the following manner. Let us fuppofe 

 two ftaves erected vertically on level ground, one towards 

 the eall and the other towards the fouth of the ftation 

 where tiie inftrument ftands in a ftate of adjuftment; and let 

 it be required to afcertain the angle fubtended at the centre 

 of the inftrument by a Hne joining thefe ftaves : in the firft 

 place, the lozenge of the vernier No. i. muft be clamped to 

 360" on the horizontal circle, and the clamping fcrew F 

 muft be releafed, fo that the upper telefcope, vernier-plate, 

 and graduated horizontal circle, may all move in azimuth 

 together, till the ftafl^ to the eaft is feen in the field of 

 view ; tliis clamp F may then alfo be made faft, and the 

 fcrew of flow motion, G, will bring the ftaff to bifeft the 

 angle formed by the fpider's lines, in which fituation zero 

 of the meafuring circle is truly placed : in the next place, 

 bring the lower telefcope, by its proper fcrew H, into pre- 

 cifely the fame fituation, and fix it there by the fixing 

 fcrew I ; then, having examined that the upper telefcope 

 has not moved from the ftafF by any accident, releafe the 

 clamping fcrew that held the vernier-plate, and turn the 

 upper telefcope, till the fecond ftaff in the fouth bifecls the 

 angle of the fpider's hairs ; which may firft be done roughly 

 before clamping, and afterwards more exaftly,by the tangent- 

 fcrew of the clamp ; then, having examined the pofition of 

 the lower telefcope again, let all the four (or three) readings 

 of the vernier-plate be put down, and take the fourth (or 

 third) part of their amount as the true angle, and fee that 



both telcfcopee have their flaws bifefling their refpeftivc 

 angular fpaces as at firft, after the meafurcs are read, and then 

 the average thus afcertained will be nearly the true angle : 

 but to prove the exadlnefs of the meafure thus taken, and 

 alfo the accurate conftruftion of the inftrument, the tcle- 

 fco])e refting in the Y's may now have its pofition reverfed, 

 and then No. 3. of the four verniers muft be clamped to the 

 point 360" of the graduated circle, and the fame oper.ition 

 muft be repeated, when aiwthcr average of four meafures 

 will be had, and an average of thefe two rcfults with oppo- 

 fite index errors, may be confidered as very near the truth ; 

 and more particularly if tliere are only three verniers ; for 

 then, as we have ftated above, the readings will be at fix 

 equidillant points of the circle, and will correft for exccn- 

 tricity as well as inequality of divifions, if any cxift. In 

 ordinary meafurements of angles, in fmall furveys of land, 

 this attention to extreme ac»uracy may be fupcrfiuous, 

 where the inftrument is well centered and graduated : but 

 where the lines to be meafured are long, the angles cannot 

 be taken with too much care ; particularly when any fide of 

 a triangle is to be determined, or checked, by the oppofile 

 angle. 



In taking an angle of altitude or of depreflion with the 

 femi-circle of the theodolite in queftion, very great accu- 

 racy is not to be expeAed, feeing there is but one vernier ; 

 but by proper attention to the previous adjuftment of the 

 level of the upper telefcope, and by ufing the telefcope in 

 the reverfed pofitions, two meafurcs will be obtained with 

 oppofite errors, that countcraft each other's effefts, fo as to 

 render the meafure true to 15". It is hardly neceflTary to 

 add, that the meafure of a vertical angle is taken by the 

 horizontal fpider's line, and that the lower telefcope is of no 

 ufe in taking fuch meafure. 



We proceed, laftly, to exemphfy the ufe of a common 

 theodolite by an aftual furvey of a fmall eftate, fuch as will 

 come within the limits of one of our plates, and by an ex- 

 planation of the manner in which a field-look is kept in 

 pradlice, and its contents transferred to the formation of a 

 map, agreeably to the moft approved methods of meafuring 

 and plotting an eftate of any affigned dimenfions. For 

 this part of our article we arc indebted to Mr. James Wad- 

 more, whofe experience and acknowledged flcill, in his pro- 

 feflion of land-fuiveyor, eminently qualify him for commu- 

 nicating the requifite information which we have referred 

 for this place, to render the fubjeft o{ furveying complete. 



" Preparatory to making a furvey with the afTiftance of 

 a theodolite, I confider it indifpenfably neceffary, (fays 

 Mr. Wadmore, ) that the furveyor fhould feel well affured 

 that his inftrument is in a perfeft ftate of adjuftment, and 

 alfo that his chain is correft, otherwife no dependance can 

 be placed upon the moft partic-ular and minute field-book he 

 could make ; for if thefe primary things are not attended 

 to, the refult of his labours, on plotting his dimenfions, will 

 turn out to be only fo much time loft, as well as that which 

 he may have beftowed upon his furvey. 



" In order to obviate thefe difficulties in fome meafure, I 

 moft earneftly recommend to the young praftitioner, that, 

 in choofing his theodolite, as well as his protraftor, cafe of 

 inftriunents, plotting fcalcs, &c. on which every thing de- 

 pends to enable him to complete his furveys with correft- 

 nefs, that he be not fparing of a fittle money in purchafing 

 thofe that are good, and can be rehed upon for accuracy ; 

 as the being furnifhcd with fuch, in the firft inftance, will 

 be the means of enfuring to him that ultimate difpatch in 

 the progrefs of his profcflion, which will lead him to tl»e 

 moft fatisfa£lory rcfults. 



" As nearly the whole of the profaflion of furveyors 



have fome material or nice diflinftiona in keeping their f^ld- 



3 O 2 books, 



