Pboceedings of the PoLYTEcirmc Association'. 877 



divers projects to bring about the sobition of the problem ; and five 

 years later M. Martens sliowed how it was possible to fix npon the 

 surface of a cylinder a succession of images received through a 

 narrow slit, by means of a movable lens. In this manner M. 

 Garella, Engineer-in-Chief of Mines, proceeded to perfect the instru- 

 ment in such a way as to render it possible that images might be 

 taken on a plane surface, and that the wdiole tour of the horizon 

 might be secui'ed. The details of the instrument were exceedingly 

 ingenious ; but when it was desired to construct a plan, long calcula- 

 tions and deductiojis were necessary, causing delay, and giving rise 

 to the possibility of errors. 



Seventeen years after the invention of Daguerre, Mr. A. Chevalier 

 stated in precise terms the problem to be solved, establishing the 

 fact that in order to obtain a plan npon paper, it was necessary to 

 construct at each station, a special protractor, formed photographi- 

 cally by the whole of the surveying signals which the instrument 

 registers automatically, according to the angular separations, as 

 observed by the eye from the station selected. This result Chevalier 

 obtained by causing a vertical lens to make the tour of the horizon 

 by means of clock-work mechanism, and tlius to secure images of the 

 signals situated around the station. These images are thrown by 

 means of a reflecting prism in rigid connection with the lens, through 

 a slit placed above a horizontal sensitive plate. The axis of this 

 slit forms a part of the movable vertical plane, which contains at 

 once the optical axis of the lens, and the center around which it 

 moves in azimuth. The slit opens automatically and without vibra- 

 tion, after the lens and the prism have already acquired a uniform 

 rotary movement. When the tour of the horizon is finished, the slit 

 closes of itself, and the whole apparatus may be turned horizontally 

 nntil the needle of a compass attached coincides with the zero upon 

 its disk ; then by opening an aperture specially constructed for the 

 purpose, the light is allowed to trace on the margin of the plate a 

 line indicating on the picture the direction of the magnetic meridian. 

 Tliis precaution serves to give the direction to the negative, and 

 allow^s the draftsman, when constructing his plan, to identify the 

 several combinations of subsequent tours of the horizon. 



In this circle of photograpic images, all the signals preser-^'e 

 between themselves the true angular separation as seen from the 

 station. In rare cases, wliere the signal is too high or too low in 

 relation to the instrument, a small lens, movable round a horizontal 



