A Treatise on the Principal Mathematical Instruments Employed in Surveying, Levelling, and Astronomy...Troughton and Simms, 1850 - 130 pages |
From inside the book
Results 1-5 of 6
Page 27
... parallax , which will be the case if , on looking through the telescope at some distant object , and moving the eye sidewise before the eye - glass , the object and the wires remain steadily in contact ; but if the wires have any parallax ...
... parallax , which will be the case if , on looking through the telescope at some distant object , and moving the eye sidewise before the eye - glass , the object and the wires remain steadily in contact ; but if the wires have any parallax ...
Page 55
... parallax , to obtain the true altitude , -subtracting the former and adding the latter ; and when the sea horizon is em- ployed , a quantity must also be subtracted for the dip , which is unnecessary when the altitude is taken by means ...
... parallax , to obtain the true altitude , -subtracting the former and adding the latter ; and when the sea horizon is em- ployed , a quantity must also be subtracted for the dip , which is unnecessary when the altitude is taken by means ...
Page 64
... parallax be applied , the result will be the true alti- tude of the centre . Observed angle Index error EXAMPLE . · 122 25 50,00 17,05 2 ) 122 25 32,95 App . alt .. Semidiameter Parallax • 61 12 46,47 + 15 46,91 + 4,00 61 28 37,38 ...
... parallax be applied , the result will be the true alti- tude of the centre . Observed angle Index error EXAMPLE . · 122 25 50,00 17,05 2 ) 122 25 32,95 App . alt .. Semidiameter Parallax • 61 12 46,47 + 15 46,91 + 4,00 61 28 37,38 ...
Page 98
... parallax ; and also making five revolutions of the screw exactly measure a five minute space on the graduated circle . For the former of these adjustments , draw out the eye - piece , d , until distinct vision of the wires is obtained ...
... parallax ; and also making five revolutions of the screw exactly measure a five minute space on the graduated circle . For the former of these adjustments , draw out the eye - piece , d , until distinct vision of the wires is obtained ...
Page 99
... parallax can be detected . Next , to examine and adjust the run ( as it is termed ) of the screw . If the run have been carefully adjusted by the maker , and no alteration made in the body of the microscope , the image of the space ...
... parallax can be detected . Next , to examine and adjust the run ( as it is termed ) of the screw . If the run have been carefully adjusted by the maker , and no alteration made in the body of the microscope , the image of the space ...
Other editions - View all
Common terms and phrases
accuracy add the log adjustment altitude artificial horizon attached axis azimuth back station barometer Beam Compasses bisect Brass bubble centre chains circumpolar star clamp co-secant coincide compass computed correct cross-wires determined deviation difference of level direct the telescope Ditto ditto divided divisions edge Electrum equal eye-end eye-piece feet fixed foot-screws glass graduated Gunter's chain half height hori horizontal limb horizontal wire inches index error instru interval latitude line of collimation longitude lower mean measured ment meridian method micrometer microscope minutes motion moved Nautical Almanac needle object observed obtained paper parallax parallel plate-screws Parallel Rulers perpendicular placed plane polar distance position protractor quantity reading right ascension round scale screw sextant shewn shews sidereal sight spirit-level staff staff-head star staves subtract sun's survey taken tangent tangent-screw theodolite tion transit TROUGHTON'S tube turning upper limb upper plate vane vernier vertical arc zenith distance zero zontal
Popular passages
Page 26 - The first adjustment is that of the line of collimation ; that is, to make the intersection of the cross wires coincide with the axis of the cylindrical rings on which the telescope turns : it is known to be correct, when...
Page 58 - The foregoing instructions for taking distances apply equally for taking altitudes by the sea or artificial horizon, they being no more than distances taken in a vertical plane. Meridian altitudes cannot, however, be taken both backwards and forwards the same day, because there is not time ; all...
Page 52 - If it require correcting, the arc will appear broken where the reflected and direct parts of the limb meet. This, in a well-made instrument, is seldom the case, unless the sextant has been exposed to rough treatment. As the glass is in the first instance set right by the maker, and firmly fixed in its place, its position is not liable to alter ; therefore no direct means are supplied for its adjustment.
Page 60 - But, what is still of more consequence, the error of the centre is perfectly corrected by reading the three branches of the index ; while this property, combined with that of observing both ways, probably reduces the errors of dividing to one-sixth part of their simple value. Moreover, angles may be measured as far as one hundred and fifty degrees : consequently, the sun's double altitude may be observed when his distance from the zenith is not less than fifteen degrees, — at which altitude the...
Page 19 - ... deviation in it is easily rectified, by releasing the screws by which it is held, and tightening them again after having made the adjustment ; or, what is perhaps better, note the quantity of deviation as an index error, and apply it, plus or minus, to each vertical angle observed. This deviation is best determined by repeating the observation of an altitude or depression in the reversed positions, both of the telescope and the vernier plate: the two readings will have equal and opposite errors,...
Page 26 - ... for determining the axis of the tube, or line of collimation. By turning the milled-headed screw, A, on the side of the telescope, the internal tube, a, will be thrust outwards, which, carrying the object-glass, is by this means adjusted to its focal distance, so as to show a distant object distinctly.
Page 53 - The amount of the index error may be found in the following manner: clamp the index at about 30 minutes to the left of zero, and looking towards the sun, the two images will appear either nearly in contact or overlapping each other ; then perfect the contact, by moving the tangent-screw, and call the minutes and seconds denoted by the vernier, the reading on the arc. Next place the index about the same quantity to the right of zero, or on the arc of excess, and make the contact of the two images...
Page 35 - Now, if the stake 6 be half way between a and c,f then ought c" — c' — (A" — A) to be equal to 2 [B" — B'— (A"— A')] ; but if not, alter the screws which adjust the diaphragm, and consequently the horizontal spider line, or wire, until such be the case ; and then the instrument will be adjusted for collimation. " To adjust the spirit-bubble without removing the' instrument, read the staff A, say it reads A'", then adding (A'"— A') with its proper sign to B
Page 18 - ... must be repeated carefully, until the adjustment is satisfactory. A similar proceeding will also put the vertical line correct, or, rather, the point of intersection, when there are two oblique lines instead of a vertical one. The second adjustment is that which puts the level attached to the telescope parallel to the rectified line of collimation. The clips...
Page 39 - AC 2AC nearly ; that is, the difference between the true and apparent level is equal to the square of the distance between the places, divided by the diameter of the earth ; and consequently it is always proportional to the square of the distance.