428 Progress in Science. [July, 



inches long, and z\ inches in diameter. The peculiarity of the action is, that the 

 hand in driving feels little or no resistance if the circuit of the larger coil be 

 of small resistance ; but it has to exert great force to maintain the same velo- 

 city in a circuit of great resistance. The driving power must therefore be in 

 some measure proportional to the/work done in the outer circuit, and when this 

 circuit is broken, and the electric action is limited to the reciprocal action of 

 the small coil and the electro-magnet, it is excessively hard to turn the handle 

 of the machine. With the same rotating speed a current approximately of the 

 same strength can be maintained in any external circuit, provided only the 

 driving power be sufficient. At a certain rate of motion, for instance, the 

 hand feels it has little to do in melting or igniting two inches of a soft iron 

 wire ^g- of an inch in diameter ; to effect the same with six inches of the same 

 wire, one hand finds enough to do, and when eight inches are in circuit, both 

 hand are needed to bring it to ignition. Even with the eight inches, little re- 

 sistance is felt till the heating of the wire ensues. When the external circuit 

 is made through the coils of an electro-magnet a few pounds in weight, it is 

 impossible to keep up a high rate of speed, as the point of saturation of the 

 magnet is never reached. But with a voltameter it is different; the limit of 

 resistance is soon reached, and the driving resistance does not rise beyond a 

 moderate amount, so that the quantity of explosive gas varies with the work 

 of one hand from 3 to 6 cubic inches only, according to the velocity of motion. 

 The principles involved in the action of the machine, which were discussed at 

 some length, seem peculiar and interesting. 



Professor A. A. Mayer, of Pennsylvania, has invented a new method of 

 fixing, photographing, and exhibiting magnetic spectra. He first coats a clean 

 plate of glass with a solution of shellac in alcohol, in the same manner as a 

 photographic plate is coated with collodion. After the plate has remained a 

 day or two in a dry atmosphere it is placed over the magnet, so that the under 

 surface of the plate just touches the magnet. Fine iron-filings of Norway iron, 

 which has been repeatedly annealed, are now sifted uniformly over the film 

 of lac. The spectrum is then produced on the vibrating plate by letting a light 

 piece of copper- wire fall vertically upon it at different points. The plate is 

 now cautiously placed on the end of a cylinder of pasteboard, which serves as 

 a support in bringing it quite close to the under surface of a cast-iron plate, 

 which has been heated over a large Bunsen flame. Thus the shellac is 

 uniformly heated, and the iron-filings sink into the softened film. When the 

 shellac is hardened it fixes the spectra. When photographic prints are to be 

 made from the plate, the heat is allowed to act until the metallic lustre of the 

 filings has disappeared, and the film appears quite transparent ; but when the 

 plate is to be used as a magic-lantern slide the heating is not carried so far. 

 Many plates have been made by this process, showing the action of single 

 magnets of various forms and of juxtaposed bars, as well as the effects of 

 electric currents led by wires through holes drilled in plates. Among the most 

 interesting are those spectra exhibiting the inductive action of magnets on bars 

 of soft iron and the interaction of magnets and electric currents. 



A lecture experiment to prove that mercury is heated while a galvanic 

 current passes through it has been devised by Dr. F. C. G. Muller. Take a 

 glass tube 6 cm. long and jabout 6 m.m. diameter ; heat it before the blow- 

 pipe, and, while soft, reduce its diameter in the centre to £ m.m., and next bend 

 it to a U-shape ; fill it with mercury, fasten it in' a clamp, and afterwards 

 dip the wires of a galvanic battery in the metal. This having been done, the 

 mercury in the narrowed portion of the tube will be observed to boil rapidly, 

 while a continuous series of sparks will be also exhibited. 



The uses to which a clock indicating correctly the same time in as many 

 different places as may be desired can be applied are manifold. In astrono- 

 mical observatories, public offices, manufactories, institutions, and for railways 

 such a clock would be of extreme utility. Sir Charles Wheatstone's latest 

 patent is for a magneto-electric clock driving sixty or seventy other clocks, 

 and dispensing with all voltaic batteries or other common causes of failure. 

 The system consists of a motor- or driving-clock and as many clocks in circuit 



