## Outline of the Method of Conducting a Trigonometrical Survey, for the Formation of Geographical and Topographical Maps and Plans: Military Reconnaissance, Levelling, Etc |

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Page viii

Clinometer " for Measuring the

Clinometer " for Measuring the

**Angles**of Slopes . - Topographical Sketches . Judgment of Distances . - Military Reconnaissance of an Enemy's Works . - Conventional Signs . CHAPTER VI . LEVELLING . CHAPTER VII . Page xi

Length of a second of a degree of latitude and longitude 16. Corrections for curvature and refraction · • 17. Reduction upon each chain's length for different vertical

Length of a second of a degree of latitude and longitude 16. Corrections for curvature and refraction · • 17. Reduction upon each chain's length for different vertical

**angles**. 18. Ratio of slopes for different vertical**angles**19. Page 11

... during the measurement , and are thus made use of : — The stations F and G are selected , so that the

... during the measurement , and are thus made use of : — The stations F and G are selected , so that the

**angles**at E may be nearly right**angles**, and the points themselves nearly equidistant from the line , and about equal to AE . Page 12

those selected on either side of it , where all the

those selected on either side of it , where all the

**angles**are measured . From A E , then , and the three observed**angles**, GE and EF are determined , from each of which in the triangles GED and DEF the side ED is obtained ... Page 13

... with reference to their relative positions ; as the nearer these triangles approach to being equilateral , the less will be the error in the calculation of the sides resulting from any slight inaccuracy in the observed

... with reference to their relative positions ; as the nearer these triangles approach to being equilateral , the less will be the error in the calculation of the sides resulting from any slight inaccuracy in the observed

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accuracy accurate Additives adjustment altitude angle apparent applied approximate ascertained axis azimuth barometer base boundaries calculated centre chapter chronometer circle compared construction contained contents contours correction course declination depression described detail determined difference direction distance division drawn earth elevation equal error evidently feet figure fixed given gives ground half height horizontal hour inches instrument intersection interval known laid land latitude latter laying length longitude marked mean measured meridian method miles minute nature nearly necessary noon object observed obtained operation parallax parallel plane plotted pole portions position practical proportion quantity reading reference refraction represented roads scale screw sidereal sides sketch spherical star stations supposed surface survey taken taking telescope temperature theodolite thermometer tion traced triangles trigonometrical true vertical zenith distance

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Page 138 - Ocean, the first thing which strikes us is, that, the north-east and south-east monsoons, which are found the one on the north and the other on the south side of the...

Page 140 - An Account of the Measurement of an Arc of the Meridian, extending from Dunnose, in the Isle of Wight, Latitude 50° 37

Page 73 - AB, aBA, the sum of the two refractions ; hence, supposing half that sum to be the true refraction, we have the following rule when the objects are reciprocally depressed. Subtract the sum of the two depressions from the contained arc, and half the remainder is the mean refraction : — If one of the points B, instead of being depressed, be elevated suppose to the point g, the angle of elevation being g AD, then * " Trigonometrical Survey,

Page 74 - BA, the sum of the two refractions ; the rule for the mean refraction then in this case is, subtract the depression from the sum of the contained arc and the elevation, and half the remainder is the mean refraction *. The...

Page 81 - ... indigo), till it nearly reaches to the necks of the bottles, which are then corked for the convenience of carriage. On setting the stand tolerably level by the eye, these corks are both withdrawn, which must be done carefully, and when the tube is nearly level, or the water will be ejected with violence ; and the surface of the water in the bottles, being necessarily on the same level, gives a horizontal line in whatever direction the tube is turned, by which the vane of a levelling staff is...

Page 112 - ... diameter. B, a sliding tube of tin, moving up and down in the pot ; the head of the tube is closed, but has a slit in it, C, to admit of the thermometer passing through a collar of cork, which shuts up the slit where the thermometer is placed. D, thermometer, with so much of the scale left out as may be desirable. E, holes for the escape of steam. The pot is filled four or five inches with pure water ; the thermometer fitted into the aperture in the lid of the sliding tube, by means of a collar...

Page 114 - Assuming 30'00 inches as the average height of the barometer at the level of the sea (which is however too much), the altitude of the upper station is at once obtained by inspection of Table I, correcting for temperature of the stratum of air traversed by table II.

Page 158 - In the orthographic projection, every point of the hemisphere is referred to its diametral plane or base, by a perpendicular let fall on it, so that the representation of the hemisphere thus mapped on its base, is such as it would actually appear to an eye placed at an infinite distance from it. It is obvious, from the annexed figure, that in this projection only the central portions are represented of their true forms, while all the exterior is more and more distorted and crowded together as we...

Page 114 - When the boiling point at the upper station alone is observed, and for the lower the level of the sea, or the register of a distinct barometer is taken, then the barometric reading had better be converted into feet, by the usual method of subtracting its logarithm from 1-47712 (log. of 30 inches) and multiplying by '0006, as the differences in the column of " barometer " vary more rapidly than those in the ''''feet

Page 14 - heliotrope," which is a piece of looking-glass, so adjusted as to reflect the sun directly to any desired point, is the most perfect arrangement. For night signals, an Argand lamp is used ; or, best of all, Drummond's light, produced by a stream of oxygen gas directed through a flame of alcohol upon a ball of lime. Its distinctness is exceedingly increased by a parabolic reflector behind it, or a lens in front of it. Such a light was brilliantly visible at 66 miles distance. (385) Observations of...