ILLUSTRATING ITALIAN SCIENCE. 889 



4560. Photograph of the " Cecchi Electrical Machine," 



formed of two discs, which are placed partially one over the other, 

 the one of caoutchouc, and the other of glass, with parallel axes. 



Prof. Filippo Cecchi, Florence. 



The Cecchi electrical or dielectrical machine is composed of two discs 

 with parallel axes. The upper disc is of india-rubber, and is supported by 

 an axis of glass ; the one below is of glass on an axis of metal. The axis of 

 the glass disc has on one side a large pulley, and the axis of the other disc a 

 small pulley, and by means of a continuous cord, not crossed, there is trans- 

 mitted to the caoutchouc disc a rotatory motion eight or ten times faster than 

 that of the glass-disc ; both the discs turn in the same direction. The discs 

 are partially placed one above another, and are very close but without touching. 

 The upper part of the caoutchouc disc passes between two arms furnished 

 with metallic points, and connected with a large sphere of brass insulated at 

 the extremity of a long glass rod. To this sphere is attached the hook of a 

 condenser or else a Leyden jar formed by a barometer-tube with very thick 

 walls. The lower part of the same disc passes before a comb of metallic 

 points, called a T-comb, which communicates with the external armature of 

 the condenser, and with two friction cushions of the glass disc, and then with 

 the ground, and also with an exciter formed by a tube of brass with a ball at 

 the end. When the discs are revolving, the large sphere becomes charged 

 with negative electricity. This machine with discs of 80 centimetres diameter 

 has given sparks of the length of 42 centimetres free in the air. 



4561. Photograph of the Universal Instrument of Luigi 



Pelli, for levelling operations, trigonometrical and graphic, by 

 being adapted to the praetorian tablet. Luigi Pelli, Florence. 



The universal instrument of Luigi Pelli serves for levelling operations, 

 because it is mounted like the most exact telescope-levels. It serves for the 

 same trigonometrical operations as the theodolite, inasmuch as its azimuth 

 circle is formed in the same manner as that of the repeating theodolite, having, 

 however, only two verniers. Moreover, this instrument is provided with ali- 

 dades, and can be applied to the praetorian tablet like the dioptric, and is 

 available for all the operations that can be performed with the same. In such 

 manner are obtainable measured and graphic angles. The alidade is placed 

 so as to coincide with the plane of the tablet by means of a central screw 

 situated in the column which sustains the telescope. A point fastened with 

 springs within the screw marks on the paper the centre of the azimuthal 

 circle. 



4562. Photographs representing various Scientific In- 

 struments in the Cabinet of Geodesy and Hydrometry, Royal 

 University of Padua. Prof. Legnazzi, Director, Padua. 



GROUP I. TELESCOPES, SQUARES, AND PRISMS. 



1. Eamsden's Dynamometer, made in the workshops of the Royal Observa- 



tory of Padua (1866). 



2. Dioptric with pointers by the Giuseppe Stefani, of Padua (1827). 



3. Squares with triangular prism, by Ertel, of Monaco (1872). 



4. Square of Wollaston, by Merlo, of Milan (1869). 



5. Model of telescope for public instruction, designed by Professor Leg- 



nazzi, and constructed by Francesco Pasini (1875). 



