THEODOLET. 



THEODOLET. 



aoj 



tion would seem fatal to the use of parallel plates where great nicety 

 IB required ; they are however very convenient and of very ready use, 

 and perhaps if the screws are strong and the observer is careful to give 

 the telescope three or four turns round in the direction he means to 

 observe, before starting, and always to move the telescope the same 

 way, serious error may be avoided. The first object observed should 

 always be observed at the end of the service, in order to see whether 

 there has been any change in the original position. If one of the 

 screws rest in a notch, perhaps the tendency to twist may be wholly 

 overcome. 



Another contrivance which is to be found in almost all theodolites 

 ia much more objectionable. The surveyor wishes to save himself 

 addition or subtraction, and requires an adjustment by which he can 

 turn the whole circle about and bring the telescope upon the first 

 object, the verniers being previously set to zero. There is, therefore, 

 a motion with a clamp and tangent-screw for this purpose, which, as 

 the clamp has usually a very short bearing, is particularly liable to 

 yield, and so to destroy all accuracy. To remedy this unnecessary 

 evil, a second or watch telescope, as it is called, is attached to this part 

 of the instrument, and brought to bear upon a well-defined object. 

 Any motion or wriggling of the zero-clamp is betrayed by the watch- 

 telescope, and when an angle is taken it must be first ascertained 

 whether the watch-telescope keeps its position, and the position if dis- 

 turbed must be restored to the zero-tangent screw, before the observa- 

 tion is finally made. In some theodolites made for the Indian survey, 

 under the direction of Colonel Everest, the zero and slow-motion clamp 

 take the form of a repeating-table, and may be so applied. It would be 

 safer to have this motion made considerably heavier than in the pat- 

 terns we have seen, and if the instrument is likely to fall into clumsy 

 hands the watch-telescope might easily be added for greater caution. 

 Such a theodolite would, so far as we'can judge, have no limit to its 

 accuracy, except that depending on the diminutive telescope. 



For many purposes of surveying it is desirable that the telescope 

 should allow of being considerably elevated or depressed, and that 

 means should be given for measuring this angle with considerable 

 accuracy. A circle, or portion of a circle, is then fixed upon the tele- 

 scope axis, and the necessary verniers and level may be secured by a 

 tail piece or otherwise to the support. If the vertical angles are to be 

 measured as accurately as the horizontal angles, the instrument becomes 

 an altitude and azimuth circle. [CiRCLE.J But such instrument* are 

 rarely applied to the measurement of terrestrial angles. The direction 

 of the meridian was determined in the Ordnance Survey by observing 

 Polaris at its greatest elongations E. and W., and taking the middle of 

 the two readings for the direction of the north. Hence, the telescope 

 required all the transit adjustments except that for azimuth [TRANSIT], 

 and was considerably elevated above the circle. Though the results 

 were upon the whole satisfactory, yet we greatly doubt the prudence 

 of ascertaining this fundamental and delicate point from such an instru- 

 ment, or of risking the steadiness of the telescope supports by raising 

 them so much above the body of the instrument. It would have been 

 better, we conceive, to have determined the direction of the meridian 

 by a series of careful transit observations, using more optical power 

 with greater steadiness, and to have kept the theodolite to its proper 

 office, that of measuring horizontal angles, greatly reducing the height 

 of the telescope supports. The great theodolite had originally a semi- 

 circle fixed to the axis of the telescope, for measuring altitudes and 

 depressions. This has since, very properly, been removed, and a whole 

 circle substitute*!. 



Where a theodolite is merely used for surveying, the telescope 

 requires only a moderate vertical range. Mr. Troughton fixed a por- 

 tion of a circle (which may be more properly called a ilice than a 

 sector) to one or two of his 12-inch theodolites, and this construction 

 is often found in other makers. The telescope is thus kept lower, the 

 instrument is firmer, and the larger radius gives the portion of the 

 circle a seeming advantage over the entire circle of smaller radius. 

 There is, however, something very unsatisfactory in a portion of a 

 circle, and we should prefer a sort of compromise, giving the supports 

 such an elevation as would allow a vertical circle of about half the 

 dimensions of the horizontal circle. If the direction of the meridian is 

 to be determined by this instrument, the supports must be at least so 

 high as to see 2 or 3 above the latitude of the place, and the vertical 

 circle may be increased accordingly. 



It is perhaps requisite to give some description of the mode of 

 adjusting the vertical circle. Where the supports are high enough to 

 allow the telescope to pass when turned round in a vertical plane, all 

 the adjustments are the same as in the altitude and azimuth circle. 

 [<!n:ri.K.] When the telescope is too long for this, the circle must bo 

 lifted out of its Y ', in order to bring the line of sight again upon the 

 object to be bisected, and then set down again. The operation is, in 

 fact, the name, whatever be the nature of the vertical arch, and the 

 adjustment i* to be effected either by altering the level or the horizon- 

 tal wire until the reading is the same in both positions of the telescope. 

 If tlio observer has a Y level, or collimator, he can set the cross of his 

 level-wire* horizontal, and this being bisected by the telescope <if tin- 

 theodolite, the vernier must be made to read zero, and the bubble of 

 the level lie liniMj>ht to the middle by its proper screws. Or if the 

 observer possess two stands (and there is a great convenience in 

 having more stands than one in surveying), he may place the stands at 



a considerable distance from each other, and, fixing the instrument on 

 one stand, aud a mark of exactly the same height as the telescope-axis 

 on the other, observe the mark, noting its elevation or depression. 

 Now, exchanging the instrument and mark, he must re-observe the 

 depression or elevation exactly as before. On drawing the figure, it 

 will be seen that if ' light move in a straight line, 90 elevation at 

 lower station = 90 depression at higher station + the angle between 

 perpendiculars to the earth's surface at each station, which last quan- 

 tity is known from the distance between the stations, and may be 

 easily calculated, that is, depression elevation = a known angle. But 

 if the zero is wrong, depressions will be increased while elevations are 

 diminished, and rice vend, so that depression observed elevation 

 observed the known angle, instead of being = 0, will be + 2 error of 

 the vernier, which may be corrected accordingly either by~the adjust- 

 ment of the level or of the horizontal wire. Or, lastly, if the telescope 

 has so much motion as that a star can be observed directly and by 

 reflexion from mercury or any other fluid, the index-error of the verti- 

 cal circle may be most accurately determined thus : Take any star in 

 the meridian, and having observed it directly, observe it immediately 

 after by reflexion. If great nicety is required, the observations should 

 be repeated alternately several times, and the partial results reduced to 

 the meridian. The mean reading between the meridian altitude and 

 meridian depression is the reading which corresponds to the horizon, 

 and the difference of this from 0, or 90, acording as the circle reads 

 altitudes or zenith distances, is the error of the instrument, which may 

 either be corrected or allowed for. This method, though very accu- 

 rate, requires some knowledge of the time, and is rather restricted 

 by the choice of stars. It is nearly as safe to observe a star not far 

 from the east or west point, first directly, then by reflexion, and lastly, 

 directly, making the contacts at following whole minutes, or at even or 

 odd minutes if the interval of a minute is not sufficient. As the stars 

 rise nearly uniformly in this part of the heavens, the mean of the first 

 aud third observations should give an altitude equal to the depression 

 observed midway. The discrepancy between these results will be the 

 double index-error as before, which may be corrected or allowed for. 

 By some of these methods the index-error of the vertical circle or sector 

 is to be found. 



In some of the older theodolites the telescope rides in Y'S at the (<>i> 

 of the vertical arch, and is reversible as a level. The horizontal posi- 

 tion of the telescope Y'S can therefore be found as in any other level, 

 and the verniers of the vertical circle set to zero when the telescope is 

 horizontal. The vertical angles measured by these instruments are 

 not, however, to be greatly depended on. They are usually greatly 

 out of balance in all positions of the telescope, except the horizontal 

 position, and therefore they make better levels than altitude instru- 

 ments. This error may be partially got rid of by having a second level 

 fixed to the instrument which is parallel- to the plane of the vertical 

 circle, and adjusted to the telescope-level when that is horizontal. If 

 this supplementary level is pretty well graduated, it will show the tilt 

 which is given the plane of the instrument by want of balance, and so 

 give the correction required. 



It may be as well to mention here that the principal adjustment 

 being that of setting the plane of the theodolite horizontal, or, more 

 correctly speaking, the principal axis vertical, any horizontal level any- 

 where placed is sufficient for the purpose, though the cross-levels are a 

 little handier. A box-level is convenient, if a stand and repeating-table 

 are used, to bring the planes nearly horizontal, and to make both ends 

 of the bubbles visible at first. 



Many surveyors give themselves and the instrument-maker a great 

 deal of unnecessary trouble by being very difficult on the chapter of 

 excentricity, which they confound with error of division. The English 

 dividing-engines, up to the present time, do not divide the circles upon 

 their centres ; and therefore it frequently happens that the point round 

 which the circle turns is not the point round which it is divided. 

 When this error is not absolutely monstrous, the only effect is that one 

 vernier gains what another loses, and that the mean of two opposite, 

 or of three, four, or more equidistant readings, is precisely the same as 

 if there were no excentricity. The advantage of a little excentricity is, 

 that it gives you the benefit of an unbiassed reading at every vernier 

 as well as the first : again, if all the verniers are recorded, it is a check 

 on the dishonest observer, who might read one vernier and set down 

 the rest. The instrument-maker must please his ignorant customer, 

 and so either hammer his circle after it is divided, which may deform 

 his work, or have an adjustment, which injures its solidity. 



In Kamsden's great theodolite, and several others which have been 

 made, the circle is read off by micrometer microscopes. Sometimes 

 the microscopes revolve with the telescope (as the verniers do in our 

 figure) ; sometimes the microscopes are fixed, and the circle revolves 

 with the telescopes, as in Kamsden's theodolite. 



Ertel of Munich has made several aitronomical theodolites in which 

 the rays entering into the telescope are reflected along the horizontal 

 axis by a prism. The observer therefore looks in at the end of the 

 horizontal axis, whatever the position of the star may be. The cyo 

 and body of the observer are more satisfactorily placed, and the sup- 

 ports are kept close and snug to the horizontal circle. The instrument 

 is well adapted to one of its principal objects, observing stars at their 

 passage over the prime vertical [TRANSIT] ; but there is some trouble 

 in finding an object when you have no better direction to look for it 



