Practical geodesy |
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accuracy adjacent angles adjusted adopted altitude angle of depression angle of elevation angular instrument argand lamp ascer ascertained atmosphere axis barometer brass bar Butser Hill calculation centre chain column construction contained arc correction cross wires degree described determined difference direction earth equal error extremities Fahrenheit feet field-book fixed fore sight give given ground Hill horizontal lines Hounslow Heath hypotenuse inch intersection Kit Hill Lansallos length logarithms mark means measured mercury method mile nearly normal contours objects observed angle obtained operations parallel pickets plane plate plotting pole pole star position protractor quantity radius reading reduced reference refraction right angles rods scale sextant sides signals spherical excess spirit-level staff standard station lines straight line surface surveyor taken telescope temperature theodolite thermometers tion traced triangles trigono Trigonometrical Survey tripod vane vernier vertical arc vertical distance yards
Popular passages
Page 209 - Telescope parallel to the Plane of the Sextant. This is known to be correct, when the Sun and Moon, having a distance of 90° or more, are brought into contact just at the wire of the telescope which is nearest the plane of the sextant, fixing the index, and altering the position of the instrument 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,...
Page 214 - Suppose, now, two observers, at distant stations, A and B, each independently of the other, to set and regulate his clock to the true sidereal time of his station. It is evident that if one of these clocks could be taken up without deranging its going, and set down by the side of the other, they would be found, on comparison, to differ by the exact difference of their local epochs ; that is, by the time occupied by the equinox, or by any star, in passing from the meridian of A to that of B : in other...
Page 27 - First, to place the intersection of the wires in the telescope, so that it shall coincide with the axis of the cylindrical rings on which the telescope turns...
Page 209 - Clamp the index at about 30- 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 perfect as before, and call the minutes and seconds on the arc of excess,*...
Page 32 - ... alone. Now the errors of observation, when numerous, tend to balance and destroy one another ; so that, if sufficiently multiplied, their influence will disappear from the result. There remains, then, only the constant error of graduation, which comes to be divided in the final result by the number...
Page 208 - ... object passes exactly over or covers its image, as seen directly ; and any error is easily rectified by turning the small screw, i, at the lower end of the frame of the glass. To examine the Parallelism of the Planes of the two Glasses, when the Index is set to Zero. — This is easily ascertained ; for, after setting the zero on the index to zero on the limb, if you direct your view to some object, the sun for instance, you will see that the two images (one seen by direct vision through the...
Page 209 - ... 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 perfect as before, and call the minutes and seconds on the arc of excess the reading off the arc ; and half the difference of these numbers is the index error ; additive when the reading on the arc of excess is greater than that on the limb, and subtractive when the contrary is the case....
Page 208 - 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. To examine the Horizon-glass, and set it perpendicular to the Plane of the Sextant. — The position of this glass is known to be right, when by a sweep with the index, the reflected image of any object...
Page 222 - ... distance, to obtain the approximate Greenwich mean time corresponding to the given distance. If the distance between the Moon and a Star increased or decreased uniformly, the Greenwich...
Page 31 - The ever varying fluctuations of heat and cold have a tendency to produce not merely ternporary and transient, but permanent, uncompensated changes of form in all considerable masses of those metals which alone are applicable to such uses ; and their own weight, however symmetrically formed, must always be unequally sustained, since it is impossible to apply the sustaining...