MECHANICS AND USEFUL ARTS. 69 



telescope is placed on a stand with screws and clamps, allowing 

 of both horizontal and vertical motion, as it may often be neces- 

 sary to give traverse to the instiTiment in order to make a connected 

 drawing of a larger area than can be included in the object-glass 

 at one view. In fact, an entire panorama can be traced, if the 

 relative position of the axis of the telescope and the surface of 

 the sketching-table are undisturbed. 



We see no reason to doubt that M. Revoil's eye-piece might be 

 adapted to the ordinary theodolite, so that any person who pos- 

 sesses one of these instruments may, at a small expense, obtain 

 a good sketching apparatus. 



The advantages possessed by the teleiconograph over the cam- 

 era lucida are manifest. The size of the image may be determined 

 at will by the person who uses the former, without any diminu- 

 tion of accuracy. We have before us a lithograph of the summit 

 of one of the towers of Notre Dame de Paris. The "croquis" 

 was taken by means of the instrument of M. Revoil, at the dis- 

 tance of about 300 metres. It is 12 inches long. A sketch, taken 

 by the aid of a camera lucida, is drawn alongside, and is only 

 one inch in length, or one-twelfth part of the linear measure of 

 the bold outline of the teleiconogram (as we suppose the new 

 likeness will be called.) Two mountain-peaks, in Provence, 

 sketched by aid of the same apparatus, show how admirably it 

 can be applied to the sketching of country. For the purpose of 

 military surveying its services promise to be of the utmost value. 



The teleiconograph insures certitude in drawing, but it does 

 not draw. It is an aid to the artist, not a self-acting substitute 

 for his eye and hand. The sharp, bold touch of a master of the 

 art of drawing will be as distinct from the feeble peddling of an 

 inferior workman, when the refracting prism is used, as when 

 freehand sketching is resorted to. The division of attention be- 

 tween the object and the copy, wiiich is often so painful, will be 

 entirely avoided by the use of this instrument. In the hands of 

 a true artist the result will be every way admirable, — exact as a 

 photograph, without the distortion of all those parts of the field 

 which are distant from the centre, and at the same time marked 

 by all the peculiarity of touch proper to the master. The camera 

 lucida, from its gi-eater portability, will still hold its own ; but 

 we shall hope to see M. RevoiPs instrument brought into familiar 

 use in this country, to meet circumstances for which it is pecu- 

 liarly adapted. — Builder ^ July 17. 



METHOD OF DETECTING POISONOUS GASES. THE GASOPHANEK. 



The "Pioneer," England, states that a discovery has been 

 made by an officer, which, if the results on a large scale are at 

 all commensurate with the experiments made on a small one, 

 may prove of great value in giving a timely indication of the 

 approach or presence of that poisonous state of the atmosphere 

 which is gcnerall}^ believed to precede cholera and other epi- 

 demic diseases. 



The gasoi>haners, or poisonous gas indicators, as the discov- 



