REPORT FOR 1897-PART II. APPENDIX NO. 10.

705

Great care should be exercised in both cases to maintain the other level (tbe one at right angles to the sensitive plate) at its proper reading in order to expose both plates in vertical plan.

After measuring CO and OA or BQ and QD, a simple proportion will give the proper reading of the transverse level, which will bring the horizon line of the vertically exposed plate throngh the two notches O and Q of the métal box.

The correct reading of the other transverse level is found by the same method, with the caméra in the vertical position, fig. 83.

Ail these operations must be executed with great care and précision, and with the help of a microscope of moderate power, as the subséquent iconometric platting of pictured points is based upon the détermination of the ordiuates and abscissæ of sueh points on the photographs, with reference to the principal and horizon lines, as a System of rectangular coordinates.

It had been assumed that the levels were placed very nearly in correct adjustment by the maker, as previously mentioned. If found too much out, they should of course be first approxi-mately adjusted by setting the métal box on a leveled plate. For this purpose the plate glass sent out with every instrument is set on the caméra base and leveled like an artificial horizon.

(3) Use of the instruments eomprised in the Canadian p ho to top ograp hic outfit. —The instruments and tripod being made as light as possible, steadiness is secured by a net suspended between the tripod legs in which a heavy stone is placed. With this device better photographs and more précisé observations are obtained, and there is no risk of the instruments (resting upon the tripod) being blown over during one of the sudden and strong gusts of wind so fr'equently encountered on elevated peaks in the mountains.

After having arrived at a triangulation station, the surveyor adjusts the transit and observes the azimuth and zénith distances of ail signais marking the triangulation and caméra stations that may be visible from his position. If accompanied by his assistant, each reads one vernier and both enter the readings in record books. After completion of the observations they compare notes. Any discrepancy that may be discovered in the recorded data is corrected on the spot.

The caméra is carried in a sole-leatker case containing also twelve fllled plate holders. When more plates are needed they (with the necessary holders) must be carried in a separate réceptacle. Taking the caméra out of the case, the leveling base, lig. 86, is screwed to it, and the caméra is then placed upon the tripod, from which the transit had been removed, without disturbing the position of the tripod 5 the shade or hood is now unfolded and attached to the hooks at the front of the caméra, fig. 82. A plate holder is inserted into the carrier, and its number is recorded upon a rough outline sketch of the view commanded by the field of the caméra image, entering also such notes as may be of value for the development of the plate and for the iconometric platting of the topography recorded upon it (by the action of the light). Having made sure that the cap is on the lens, the slide is withdrawn from the plate holder and the plate is brought into contact with the frame of the métal box by turning the screw G, figs. 84 and 85, devised for this purpose. The surveyor now turns the caméra in azimuth until the lines on the upper face of the wooden casing show that it is properly directed or oriented to include the panorama section to be photographed between the lines, the field of view coinciding with the outline sketch bearing the number of the plate holder in the caméra. Sighting along the converging lines, shown on the side face of the wooden caméra casing, he can assure himself whether the view on the image plate reaches high or low enough. If it does not, he will put the longer dimension of the caméra upright, unless the caméra was already in that position. He levels carefully, in the manner previously described, and exposes the plate. Whenever the sun shines inside of the front hood it should be shaded off during the exposure of the plate by holding something above the hood. Under no circumstances should the snn be permitted to shine upon the lens.

Every evening, after returning to the survey camp, the surveyor replaces the exposed plates in his dark tent by new ones, using a ruby-colored light. He also marks the exposed plates in one corner, before removal from the holder, with his initiais, the number of the dozen and of the plate (the same number as given to the corresponding outline sketch), using a soft lead pencil for this purpose; e. g., IV, 5, means plate No. 5 of the fourth dozen, or the forty-first plate. The exposed plates are now placed into a double tin box, fig. 93, which can be closed hermetically, and which will float when filled with two dozen plates, should the same be accidentally thrown into 6584---------------45

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