may extend over many degrees of latitude and longitude, and even include a large region as for instance, that of India, of Germany, France, or the British Isles. The utmost precision being necessary in such work, the general management of it is entrusted to scientific men, with whom rest the initiation or adoption of improved methods, instruments, and appliances; while the carrying out of the detailed routine work, its checking and superintendence, is usually delegated to inferior men that happen to be naturally well fitted to adhere to a rigid system of work not requiring the exercise of much judgment or discretionary power. These systems and appliances are fully described in books devoted to that special subject, which is beyond the scope of this work; they will hence be merely referred to here in so far as they serve as guides, or afford indications useful in ordinary practice. The least perfect type of Survey-work is termed Military Surveying. It is conducted on the principles of ordinary practice, but in a rough manner, mostly with pocket-instruments, aided greatly by good guess-work; the permissible amount of error in it is large, while extreme rapidity is of paramount importance. For such surveying no additional training or special instruction is necessary, although a knowledge of strategical requirements is a valuable adjunct.

All ordinary general survey-practice lies between these two extreme types, and may be thus classified on the first-mentioned principle.

1. District Surveys, of extended tracts of country. These being of large scope, and requiring great precision, are carried out in style and method after the model of operation originally adopted in Great Trigonometrical Surveys, although less exact and cumbrous

instruments and appliances are used, and a very much less amount of refinement and exactitude, as well as time, is expended on them.

2. Parish and Estate Surveys, being more limited in scope, require less exact instruments and appliances, but follow the general principles adopted in the filling in of Great Trigonometrical Survey work.

3. Town Surveys, which ordinarily cover about the same extent of land and range of limits as those of the last class, differ from them in requiring greater exactitude, as they have to be plotted to a much larger scale, and in involving a greater amount of labour.

4. Engineering Surveys. These having for their special object the future location of some work of public or private utility, such as a railway, canal, waterworks, drainage works, or factories, are generally confined either to a long narrow strip of country or a small tract of country or plot of land; the amount of exactitude necessary being liable to vary all over the ground in accordance with its bearing on future requirements and the nature of the intended works.

5. Route-Surveys have in view the limited object of producing a map or plan of a route taken by an explorer or traveller either by land or sea; in the latter case it is usually termed navigation, and consists in laying down the ship's course: the position of natural and artificial features of the country, or of detached points of land, in the immediate vicinity of the route, are also recorded in these surveys.

6. Mining and Subterraneous Surveys are limited by the extent of actual or possible future mining operations, tunnels, passages, or works of construction; they generally involve a record of position or distribution of

minerals in the soil, and of other geological and hydrogeological data.

7. Hydrographic and Marine Surveys, of rivers, lakes, estuaries, marshes, and coasts, comprise not only a general survey of the land in the immediate neighbourhood of the water, as it may appear under various conditions of flood and tide, but also such a record or delineation of the subaqueous land, of currents, soundings, and depths, also in some instances, of quantities of water, that the subaqueous formation under any conditions may be as thoroughly described in plan as the former. Such surveys are subdivided into the two classes of Nautical and Engineering Hydrographic Surveys, the former being of greater extent, the latter of greater precision but within narrower limits.

All such surveys may be perfectly independent, and may be so executed as to suit any required scale in the final plan; but in practice they are frequently based on, or joined on to, the results of previous surveys in existing maps and plans; in such a case it is best to adopt a scale that has some convenient ratio to that of the existing plan. A list of scales for reference will be given in Section 1 of Chapter II.

The following are the operations which either separately or in combination are practised in surveys of every description.

1. Direct measurement of distance, and of height and depth.

2. Angular measurement and taking magnetic bearings,

3. The calculation of distances, heights, depths, and angles.

4. The selection and demarcation of survey points

and lines.

5. Recording results in such a form and manner that a draughtsman may utilise the results without doubt or difficulty.

6. Astronomical observations for determining time, latitudes, longitudes, and azimuths.

The various means and appliances used in direct and angular measurement on surveys will be mentioned in the two next sections, the special principles involved in their construction and use, the limits of their application, and the preferable methods of manipulation and adjustment, occasionally entered into in detail; the corrections to be applied to results of instrumental observation, under various special conditions will also be fully explained and deduced; but definitions of terms, descriptions of easily visible instruments and appliances, and practical directions for commonplace manipulation, have generally been excluded from this work as unnecessary.

The two following sections will treat of the selection and demarcation of survey-points and lines in the field, and give a collection of formulæ commonly used in survey calculations; the less frequent or more special formulæ will be found attached to their appropriate subject or instrument. The subject of astronomical observations, the adjustments and corrections to be applied to the Transit instrument, and the necessary formulæ for all such purposes, are included in the Part devoted to Route-Surveys. The ordinary forms and modes of recording survey operations are fully exemplified in a series of practical field-records suited to surveys of every sort, which are collected at the end of the book, though their corresponding sketches or reduced plans are interspersed with the text bearing on the subject.

minerals in the soil, and of other geological and hydrogeological data.

7. Hydrographic and Marine Surveys, of rivers, lakes, estuaries, marshes, and coasts, comprise not only a general survey of the land in the immediate neighbourhood of the water, as it may appear under various conditions of flood and tide, but also such a record or delineation of the subaqueous land, of currents, soundings, and depths, also in some instances, of quantities of water, that the subaqueous formation under any conditions may be as thoroughly described in plan as the former. Such surveys are subdivided into the two classes of Nautical and Engineering Hydrographic Surveys, the former being of greater extent, the latter of greater precision but within narrower limits.

All such surveys may be perfectly independent, and may be so executed as to suit any required scale in the final plan; but in practice they are frequently based on, or joined on to, the results of previous surveys in existing maps and plans; in such a case it is best to adopt a scale that has some convenient ratio to that of the existing plan. A list of scales for reference will be given in Section 1 of Chapter II.

The following are the operations which either separately or in combination are practised in surveys of every description.

1. Direct measurement of distance, and of height and depth.

2. Angular measurement and taking magnetic bearings.

3. The calculation of distances, heights, depths, and angles.

4. The selection and demarcation of survey points. and lines.