ELECTRICAL APPARATUS. 



305 



diameter, closed air-tight at both its ends by Fl 's- 362 - 

 two brass caps ; a small glass tube, open at 

 both ends, the lower one bent at a right angle, 

 passes through the bottom cap, and enters the 

 water contained in the lower portion of the 

 large tube. Through the middle of each of the 

 brass caps a wire is introduced, terminating in 

 a brass knob within the tube, and capable of 

 sliding through the caps, so as to be placed at 

 any distance from each other. If the two knobs 

 be brought into contact, and a Leyden jar 

 discharged through the wires, the air within 

 the tube undergoes no change in volume ; but 

 if the knobs are placed at some distance from 

 each other when the jar is discharged, a spark 

 passes from one knob to the other ; the consequence is a 

 sudden rarefaction of the air in the tube, shown by the wa- 

 ter instantaneously rising to the top of the small tube, and 

 then suddenly subsiding ; after which it gradully sinks to 

 the bottom of the tube, the air slowly recovering its original 

 volume. Price, $3.00. 



Cavallo's Rain Mec- Fig. 363. 



troscope (Fig. 363), repre- 

 sented in the cut. A is a 

 strong glass tube, about 

 two feet and a half long, 

 having a tin funnel cemented to its extremity, which funnel 

 defends part of the tube from the rain. The outside surface 

 of the tube is wholly covered with sealing wax ; c is a piece 

 of cane, round which brass wires are twisted in different 

 directions, so as to catch the rain easily, and at the same 

 time to make no resistance to the wind. The cane is fixed 

 into the tube, and a piece of wire proceeding from it, goes 

 through the tube, and is terminated by a ring, upon which 

 a pair of pith balls are suspended. This instrument is sus- 

 pended by the side of a window frame, with the funnel pro- 

 jecting outwards, while the pith balls are preserved dry 

 within. Price, $2.50. 



Cavallos Bottle Electroscope (Fig. 364, next page) is 

 formed by two silver wires, each carrying at one of its 



