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626 station need not, however, be a trigonometrical point, provided that its trigonometrical position can be measured from the photograph. To use the photographs for plotting the detail, from each camera station A draw, at its correct angle, the central line of view. Along this line draw Ax, equal to the focal length of the lens; through x draw a line at right angles to A x and plot from * the projections of the distant points, as measured on the horizon-line of the photograph. The intersection of rays from A to the points so obtained, with rays to the same objects from other stations, will give their positions.

It is an almost universal rule of photographic surveying from the ground to maintain the photographic plate in a vertical position, because any inclination of the perspective plane of the plate adds difficulties to the plotting.

A supplementary order of triangulation is usually added contem- poraneously with the field work of photogrammetry, both to fix camera positions and to add a few bearings and vertical angles from which the photographic data may be checked.

For many years cameras specially designed for surveying work have been available, a good example being the Bridges-Lee photo- theodolite. The essential features of these are, that the focal length should remain constant (for which purpose the sensitized surface of the plate should be pressed firmly against the frame of the camera) , that the position of the optical centre, and of the horizon and prin- cipal planes should be deducible from marks on the plate, and that the lens should be free from distortion and aberration. It is also important to provide levels by means of which to ensure the vertical- ity of the plate. The later photogrammetric cameras are either interchangeable with transit theodolites on the same portable stands, carry eccentric telescopes, or else combine the two instruments by substituting a telescopic lens for the usual photographic lens, and by inserting an eyepiece in the back of the camera. The field work demands a high standard of topographic training, for it is not easy to select the minimum number of views sufficient to cover the coun- try whilst leaving no gaps. Valleys and low-lying areas constitute the main difficulty. One or two stations per diem are all that have been occupied by the same party in the photogrammetric surveys of the Canadian N.W. frontier, whilst a supplementary triangulation was carried on concurrently.

The office work takes two or three times as long as the field work and consists in plotting positions, calculating heights, and drawing contours from data measured on the developed plates.

The picture trace will naturally assume the form of the arc of a circle if a panoramic camera is used such as that employed during 1910-6 by Mr. J. W. Bagley in Alaska.

There are several plotting devices on the market, such as the perspectograph, but they have not been largely employed. On the other hand vanishing scales and perspectometers (grids showing the perspective on a vertical plane of a series of squares on a hori- zontal and lower plane) can be readily constructed.

Stereo-photo Surveying. The most recent development of photo- graphic surveying consists in the employment of the stereoscopic principle. The stereoscope as a toy has long been known, but Dr. Pulfrich of the firm of Zeiss of Jena, and Col. von Hiibl of the Aus- trian military ^geographical service, conceived the idea of applying the stereoscopic principle to the service of exact surveying. Other pioneers in 1907-8 were the late Capt. F. V. Thompson, R.E., and Mr. Conrady. In 1913 Mr. G. Muller carried out a successful stereo- photo survey for part of the proposed Hankow-Ichang railway. In order to carry out a normal photographic survey successfully it is necessary to arrange for stations far apart and for intersections of some 30 degrees or so. But in stereo-photo surveying two stations can be occupied on the same hill-top and their distance apart need only be some 50 to 300 feet.

In the simplest case let two vertical photographic plates be ex- posed from two points, say 100 ft. apart; let the plates be in the same plane and their centres on the same level. Then if these plates are put into a stereoscope provided with a system of lenses and prisms such that the eyepieces are brought to a convenient distance for seeing, we shall clearly get a very much magnified stereoscopic effect as_ compared with what is obtainable with the naked eyes. In the diaphragm of each eyepiece let there be a similar movable mark, or line on glass. On looking through the eyepieces the marks in question will appear as a single mark floating in space, and by vertical and horizontal adjustments this mark can be made to touch any given object in the picture. We have, thus, a means of measuring small parallaxes and vertical angles, and these can be read off graduated micrometer heads.

A stereo-comparator as above described gives angles from the centre of the plate, distances and vertical angles; but the reading is laborious and the map has to be constructed point by point.

In 1907 Lieut, von Orel, of the Military Geographical Institute of Vienna, attempted the construction of a machine which should quasi-automatically draw the map, and in 1909 such an instrument was made by Zeiss of Jena. A further model of 1911 permitted the automatic drawing of contours and the outline of detail. The in- strument _is called the stereo-autograph ; several have been made and are in existence in Austria, Germany and France. A stereo-auto- graph is, of course, an expensive instrument and requires a skilled operator and good plates of even density. But, given these condi- tions, practical results have been obtained and the method is one

to be reckoned with in the future. Generally with stereo-photography we are not limited to a country with marked features, as is the case with normal photo-topography. Provided that the view is clear, gently undulating or flat country can be as well surveyed and con- toured as a mountainous region. The method has some obvious applications, but it is useless in forest-clad country or in towns and its value largely depends upon good view points. The old, photographic surveying has as its chief field of usefulness a well- marked mountainous region. The new is not so limited but its r&le has not yet been fully determined.

Bibliography of Photographic Surveying. In 1895 Mr. E. Deville was able to quote the titles of 26 works on photographic surveying; in 1911 Dr. Pulfrich in his Stereoskopisches Sehen und Messen men- tions 276 works, chiefly in German, on stereo-photography alone! The following books may be recommended: Photographic Survey- ing, E. Deville, Ottawa, 1895; Hints to Travellers, vol. I. R.G.S., 1906; The Use of the Panoramic Camera in Topographic Surveying, James W. Bagley, Washington, 1917; Revue Generate des Sciences, March 1914, Paris, for stereo-photo-topography.

Longitude by Wireless Telegraphy

The chief technical difficulty which explorers and surveyors in new countries have hitherto experienced has been in the determination of longitude in regions unprovided with a telegraph system. This applies to almost all the unexplored, or little explored, parts of the world. Since 1910, however, the great advance made in the transmission of signals by wireless telegraphy has completely done away with this source of difficulty and error. I Wireless " receiving " sets are now made of a very portable character; so much so, that one mule or one porter can carry the whole apparatus. Frequent practical use is being made of this method of obtaining time signals, as the following instances will show. In the year 1912 Comm. Edwards fixed positions during the , Bolivia-Brazil boundary commission by wireless signals from Washington and intermediate stations; in 1913-4 Cav. Dr. Filippo de Filippi in an expedition to the Karakoram used wireless signals from Lahore and from Italy; Major A. J. ' Woodroffe in 1913-5 determined longitudes on the Peru-Brazil boundary commission by wireless signals sent from Senna Madu- reira, Brazil; in 1914-7 the French explorer, Lt.-Col. J. Tilho, used wireless signals from Paris to determine longitudes in his explorations of Tibesti, Borku, Erdi, and Ennedi; in 1917 Capt. A. J. Bamford determined the longitudes of Bagdad and Kermanshah by wireless signals from Fao, which had ' previously been connected with Basra; in 1910-20 the American traveller, Dr. A. Hamilton Rice, made use of wireless signals from Annapolis, Washington and Darien, to determine longitudes during his Amazonas expedition. Fig. 6 illustrates the wireless receiving set used by Dr. Hamilton Rice in 1919-20; it was designed by Mr. J. W. Swanson and Mr. P. F. Godley, and was found quite satisfactory and very portable.

It is safe to say that, in future, no properly equipped exploring expedition will be without its wireless receiving set. The designs of these sets will change from time to time and, no doubt, improve- ments will be made; but the method has proven to be thoroughly practical, and the extra amount of transport required is already of a negligible character. One of the greatest difficulties of the explorer has thus been removed.

Levelling

Since 1910 much progress has been made in the development of a sound system of levelling, especially with regard to pre- cise, or geodetic levelling, i.e. that levelling which provides the framework on which all national levels depend. The now defunct International Geodetic Assn. laid down some wise rules on the subject of the precision of work of the highest standard. The admirable treatise of M. Ch. Lallemand, Nivellement de haute Precision, marked a great advance on previous text-books; and the production of the modern geodetic levelling instruments of France, the U.S. and Switzerland afforded the means of greatly increasing the accuracy of observation. To this should be added the introduction, by the Ordnance Survey of the United Kingdom, of a specially devised kind of permanent bench-mark, which did away with a weak element in the old levelling, the instability of the ground marks.

Levelling Instruments. As a type of the instruments in use for levelling of high precision the level designed by Dr. Wildt, and made