to adjust the telescope parallel to the bubble. The smaller key is used to bring the webs into collimation, and applies to squares in the collar M. K and L are rackwork adjustments, the former to bring the webs, and the latter the object, into focus. The Adjustments. After setting the instrument as nearly level as the eye can detect, by shifting the three legs of the staff, so as to save wearing out the screws as much as possible, proceed to test the adjustments, and to put them right when necessary; for it must be borne in mind that all portable instruments of any kind are liable to derangement by carriage, and should therefore be examined carefully before use, so as to escape error in observation. There are four adjustments to attend to, first, the setting upright of the vertical centre; second, adjusting the level horizontally to the centre, which is done at the same time; third, the collimation of the telescope; and fourth, adjusting the optical axis of the telescope parallel to the level, and consequently at right angles to the vertical centre. To adjust the Centre and Level.-Unclamp the screw D that fixes the centre, and the instrument will move freely round, turn it until the level is in a line with two of the screws of the tripod C C, and make the bubble of air in the level stand central, by moving the screws in the tripod whichever way is necessary. Then move the instrument half round on its centre, and if the bubble of air remains central, the adjustments are correct, but if it moves either way it must be brought back half way by the screw in the tripod, and the other half by the screw in the level at F, to which the larger key applies, but which is purposely kept separate from the instrument to prevent any accidental interference during observation. Then turn the instrument a quarter round, or over the third screw in the tripod, and by its means bring the bubble of air again central, and if it remains so while the instrument is slowly turned a complete circle all is well, but if not, the operation must be repeated until the correction is complete. To collimate the Telescope.-Direct the telescope to some distant object, the edge of the levelling staff will answer the purpose very well, and adjust the webs to focus by the milled head K, then adjust the object to focus by the milled head L. Particular attention must be given to this, so as to avoid parallax, the presence of which is calculated to produce serious inconvenience. To ascertain if it exist, move the eye slowly from side to side while looking through the telescope, and if the webs and object wander, the adjustment is not correct, and must be made so by turning the two milled heads K and L until the webs and the object are distinct and stationary. When this is accomplished, bring one of the cross webs to intersect the edge of the staff, and clamp the centre by the screw D. Turn the telescope half round in the Y's, and if it intersect the same object the collimation is perfect, but if not, one half of the error must be corrected by the smaller key, which moves the webs, and applies to squares in the collar M, and the other half by the tangent screw F. The other web is set right in the same way, and the operation must be repeated until both webs intersect the same object as the telescope is moved round. To adjust the Optical Axis of the Telescope.-While the centre is clamped for collimation, turn the telescope round in the Y's until one of the webs is horizontal and coincides with a division on the staff. This may be easily done by one of the tripod screws, as it is not necessary that the instrument should be levelled during this operation. Then reverse the telescope in the Y's, unclamp the screw D, and turn the instrument half round, when the same object will be brought into the field of view. If the same division does not coincide with the same web, half the error must be corrected by the moveable Y, to which the larger key applies at I,, and the other half by means of the tripod screw. When this by reversion is found to be correct, the instrument is ready to commence operations. It will be seen by the foregoing, that the advantages levels of this description possess over others, are principally as follows. There is not the slightest strain in any part of the instrument, therefore it is not likely to change during observation. The adjustments being all free, are much more agreeable to use, a greater saving of time is obtained, and the annoyance of clamping and unclamping the screws, necessary when made with parallel plates, avoided. The bubble being fixed upon the solid limb is less liable to derangement than when on the telescope, and moreover, is always under observation in whatever position the telescope may be. The spring adjustment to the webs is quicker in operation, and infinitely more delicate, than the counter-pulling screws, and the squares, which receive the key, being below the surface, cannot be disturbed by anything when the key is removed. The rackwork adjustment for bringing the webs into focus is also an advantage over the sliding adjustment, which frequently, from exposure, becomes so stiff in its motion as to make it difficult to bring the webs into focus, besides the distortion to delicate parts of the instrument such a strain is calculated to produce. INDEX. ABERRATION and aplanatism of lenses Achromatic eye-piece, 73. Altitude and azimuth instrument, de- Angles, method of setting off, 16, 39; Area, of a drawing, reduced or enlarged, Astronomical triangle, 158; table of for- Barometer, portable, improvements upon, Barometric formula, 175. Beam, callipers, 176; compasses, Swiss, 178. Bench marks, 111. Bisection of a straight line, 36. Boulenge's chronscopes, 176. Bow compasses, 4. Bow sextant, described, 117; its parallax explained, 119: and eliminated, 121. Bregent's chronoscopes, 176. Callipers beam, 176. Camera obscura, 89; lucida, 90. Cassini, his aerial telescope, 81. Cassegrainian telescope, 84. Chords, line of, constructed, 14; uses of, 16. Chromatic dispersion of light explained, Chronoscopes, 176. Clamping screw, 47. Collimator, description of, 162; use of, 162. Compasses, 1; beam, 48; bow, 4; hair, Compass, prismatic, 115. Content of a drawing reduced or enlarged, 5; solid, measured, 30. Copying drawings, 52. Course, method of setting off a, 16. Hadley's sextant described, 137; its ad- justments, 139; its index error deter- Herschel, his telescope, 84. Huyghens, his aerial telescope, 81; his lypsometrical instruments, improve- Johnson's deep sea thermometer, 177. Land chain, 92. Lenses, various forms of, and effects pro- Level, spirit, 96; the Y level, 97; its ad- Levelling, remarks on, 106; for sections, Levelling instruments, 170; staves, 177. Linear measures, table of, 92. Pricking point, 8. Prism, defined, 67; its effects upon light Prismatic compass, 115. Proportion, 29, 36, 56. Protracting scales, 14. Protractors, upon plain scales, 33; circu- Radius found, from the length of the sine, note. Reflecting circle, described, 141; direc- Rhumbs, line of, 16, 17; sine rhumbs, 28; Scales, of equal parts, simply divided, 9; Scales, improved, 177. Screw, clamping, 47; tangent, 47. Sea bottoms, instruments for obtaining Secant, to any desired radius, found, 41. Sextant, box, 117; Hadley's, 137. Sines, scales of, 14; to any desired ra- Solid contents found, 30; of squared tim- Square, of a number, found, 57; root, 6; Steel gauges, 178; tape, graduated, 173. Superficies, or area, measured, 29. T square, 131 Tables, giving the correction to be sub- Tape, graduated steel, 178. Tangents, scale of, 14; to any desired ra- 84; Cassegrainian, 85; Herschelian, 85; Transit instrument described, 144; its Triangle, isosceles, having each of the Troughton, his level, 104; his reflecting Vernier, described, 45; scale, 11. Wholes and halves, 4. Y levels, 97. THE END. PRINTED RY JAMES 8. VIRTUE, CITY ROAD, LONDON. |