A Treatise on the Principal Mathematical Instruments Employed in Surveying, Levelling, and Astronomy: Explaining Their Construction, Adjustments, and Use : with an Appendix, and Tables

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Troughton and Simms, 1844 - 130 pages
 

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Page 54 - ... to make the objects appear on the other wire ; if the contact still remains perfect, the axis of the telescope is in proper adjustment ; if not, it must be altered by moving the two screws which fasten, to the up-and-down piece, the collar into which the telescope screws. This adjustment is not very liable to be deranged.
Page 59 - 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, therefore, that can be done is, to observe the altitude one way, and use the index error ; but, even here, you have a mean of that altitude, and this error taken on three different sextants. Both at sea...
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 head...
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 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.
Page 56 - ... say, as the difference of times between the observations is to the difference of their altitudes, so is the difference between the time that the first altitude was taken and the mean time, to a fourth number; which, added to, or subtracted from, the first altitude, according as it is increasing or decreasing, will give the altitude reduced to the mean time.
Page 38 - The best constructed levelling staff* consists of three parts, which pack together for carriage in a neat manner, and, when opened out for use, form a staff seventeen feet long, jointed together something after the manner of a fishing-rod. The whole length is divided into hundredths of a foot, alternately coloured black and white, and occupying half the breadth of the staff; but for distinctness the lines denoting tenths of feet are continued the whole breadth, every half foot or five-tenths being...
Page 15 - A Treatise on the principal Mathematical Instruments employed in Surveying, Levelling, and Astronomy ; explaining their Construction, Adjustment and Use, with Tables.
Page 70 - ... h. The whole of this apparatus is movable with the telescope, and when the axis is reversed, can be attached in the same manner to the opposite standard. Near the eye -end, and in the principal focus of the telescope, is placed the diaphragm, or wire-plate, which, in the theodolite or levelling telescope, need only carry two cross wires, but in this instrument it has five vertical and two horizontal wires. The centre vertical wire ought to be fixed in the optical axis of the telescope, and perpendicular...

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