THEODOLITE. 



t)«ok., aHfing cither from the peculiar methods they have 

 bcc-ii taught, the inftrumcnts ihcy ufe, the methods the more 

 experienced have been led to adopt ; from the more enlarged 

 and ei.li-jhtencd views the)- may have taken to afcertain the 

 bell poffible manner of facilitating the defircJ objed, thatot 

 making a corred furvcy ; or from the move improved flate 

 of mathematical fcicncc; Itruft I may be permitted to ofter 

 the following mt-thod of taking dimenlions, the form of 

 keeping a field-book, and the method to be adopted in 

 plotting the fame. Having made ufe of the theodolite, and 

 endeavoured, by pradical experience, to avail myfelf of its 

 moft beneficial fervlccs, I have no doubt but the fpeci- 

 men I now fubmit will be moil readily comprehended, and 

 the moft intricate and difficult furveys accompliftied by the 

 fame means ; viz. with the aflillance of the theodolite, where 

 other methods, by poffibility, may be found partially (if 

 not wholly ) to fail. 



« On commencing a furvey, I have always found it necef- 

 fary to look out for an intcUigent labourer, well acquainted 

 wth the locality of the neighbourhood, with whofe aflill- 

 ance, and another to carry the theodolite, I ufually com- 

 mence ; invariably taking the chain myfelf after the leader : 

 and here it may not be amifs to remind the young practi- 

 tioner, that, in following, he muft be moll particular in 

 direfting his chain-leader in a ftraight line, otherwife the re- 

 lative bearings of the Ilations forwai-d, taken by the theo- 

 dolite, will be rendered incorreft, and the protractor in 

 plotting will not fail to convince" him of his error, to his 

 great chagrin and difappointment. I muft alfo remind him 

 of the neceflity of keeping his chain-hand, and alfo direfting 

 that of his chain-leader to be kept as near the furface as 

 conveniently can be done, and the chain properly ftretched, 

 by which means he will obtain more coiTeft lengths in 

 mealuring from ftation to ftation, infuring thereby the 

 greater accuracy in his furvey (particularly if it be large) ; 

 as he will very early find, in the courfe of his praftice, that 

 en-ors, when once begun, will rapidly increafe ; to prevent 

 which, it wiU be found very convenient to plot every day's 

 work on his retmn home, before he commences another, 

 when, if any eiTor is found, it can be reftified on the follow- 

 ing day. 



" Many other neceflary cautions might be given to the 

 young furveyor, which by fome may be thought fuperfluous, 

 but the following I cannot help recommending to his atten- 

 tion ;■ i/iz. that of being exceedingly particular in marking 

 and defcribing his exterior boundaries, having myfelf, more 

 than once, feen litigation prevented by the produftion of 

 correA furveys, where the boundaries of eftates in difpute 

 were clearly defined. And let him not be fearful of taking 

 too many offsets, or dimenfions ; for by trufting obferva- 

 tions neceiTary to his furvey to memory, he cannot but 

 fail to omit fome ; whereas by a copious field-book, not even 

 the flightell bend in a fence, or objeft of any fort, can pofli- 

 bly be omitted. Above all, let him feel well fatisfied of the 

 corrcdlnefs of his chain ; to enable him to do which, he fhould 

 always have a fpare one, on a large furvey, to correcl by ; 

 as it is well known that accidents will happen by the break- 

 ing of the fame, by lofs of rings, &c. ; and the offset ftaff, 

 from being too (hort, cannot be depended upon for this 

 purpofe. 



" Having faid thus much, I now proceed to fhew the 

 method I have praftifed in furveying with the theodoUte ; in 

 doing which, I fiiall firft premife that I have always con- 

 fidcred it advantageous to take a view of the eftate to be 

 furveyed, by doing which, I have been enabled to avail 

 myfelf of ftudying the bell ftations, and to judge how the 

 bufinefs may be accompliftied with the grcateft facility. 



" The theodolite which I have been accuiloijied to ufe 



is a very good one, made by Cary, about five iitches 

 diameter on the limb, which is finely graduated, with a 

 vernier reading minutes, and with degrees numbered from 

 I to 360 ; it has an achromatic tclcfcope and vertical arc, for 

 the purpofe of afcertaining the angles of elevation and de* 

 preffion in hilly furveys, on one fide, and the correfponding 

 deduAions to be made in the chain lines on the other, and 

 moves by rack-work. 



" The method adopted by me, after fome years' praftice, 

 in keeping a field-book, perhaps may be thought Angular, 

 namely, that of commencing at the bottom of the la(l page 

 of the fame, and working upwards therefrom ; but it wiU 

 readily be feen, on reference to the accompanying field-book 

 in Plate X. to be the beft way, as in proceeding, you meet 

 the objefts you have to defcribe in fucceflion as you advance, 

 and it affords a much greater facility in laying off the off- 

 sets from the chain-lines, than in the other way. 



" It is well known that the needle is frequently affedled, 

 in the firft place, by the atmofphere, and liable to get out of 

 order ; in the fecond, the chain being made of iron as well 

 as the arrows, and their not being moved a fufficicnt diftaince 

 in general, when the theodohte is planted for taking an 

 angle, it may thereby be very materially affedled ; in the 

 third, articles of iron or fteel may be about the perfons 

 attending the furvey, which may have the fame effetl ; in 

 the fourth, it may not be found improbable, from the 

 very nature of the ground on which a furvcy may be carried 

 on, that veins of iron, or metallic ore, may exift under the 

 furface, which will not fail to influence the polarity, to 

 the utter detriment of afcertaining the true bearings ; and, 

 laftly, without a vernier, the angle cannot be read with fuffi- 

 cient accuracy. 



" Trufting that the foregoing remarks wiU fufiice to con- 

 vince thofe, who may be carrying on furveys by the needle 

 alone,of the danger of trufting to this method, as well as to pre- 

 vent the ftudcnt in the profeffion from fo doing, I now proceed 

 to (hew, that by taking the included angles of the furvey 

 upon the limb of the theodolite, and by carefully noting 

 them in the field flcetch, (as feen in the plate,) no fuch danger 

 can pofTibly exift. The following problem will give fuffi- 

 cient proof of the accuracy of this method of meafuring the 

 internal angles of any geometrical figure, whatever may be 

 the number of its fides ; viz. double the number of fides, and 

 multiply them by 90° ; and then, if 360° be fubtradled 

 from the produft, the remainder will be the fum of all the 

 internal angles ; for if we fuppofe lines drawn from every 

 angle to one common point in or near the middle of the 

 figure, there will be as many triangles as there are fides, and 

 the fum of all the angles at the apex, or point affumed, will 

 be 360°, as will appear when circumfcribed by a fmall circle : 

 therefore, as every triangle contains 1 80°, the amount of all 

 the triangles wiU be as many times iSo^ (or 90° x 2) as 

 there are bafes, when diminifhedby 360°. In our example, 

 tlie furvey is bounded by five chain lines, with as many in- 

 cluded angles, and therefore the amount of the angles will 



be 5 X 90° X £< — 360° ; or 900° — 360° = 540°, agree- 

 ably to the fubjoined meafurements taken in the field ; viz. 



At O 2 - - 85° 14' 



O 4 - - 88 18 



f 60 4l 



1+74 2oi 



" 1+ 71 16]: 



©I - - 87 12 



O 6 

 O 10 



Total fum 



540 



Havirj 



