Till. LIGHTHOUSES, &C. 457 



The sixteen vertical lenses are contiguous, and are each composed of a 

 single element, about 0-12 wide, the curve of which has been calculated so as 

 to give with the electric light an horizontal divergence of three degrees seven 

 minutes. The duration of a flash is, accordingly, of about five seconds, and 

 the interval between the end of a flash and the beginning of the following 

 one is 25 seconds. 



The maximum intenseness of the flash rises to about 60,000 burners,, 

 assuming at the focus an electric light of 200 -burner power. 



The light is produced in this apparatus, as in the lighthouses, with electric 

 light, established on the coasts of France, by means of a Serrin regulator and 

 an electrical machine of the Compagnie 1' Alliance. 



Experiments have been made with the Serrin regulator at the lighthouse 

 dep6t since the year 1860. A model on a large scale has been constructed 

 especially for the lighthouse service, and has always given good results. The 

 regulator exhibited is a counterpart of this model. 



The electro-magnetic machine has been, as is well known, designed by MM. 

 Nollet and Joseph Van Walderen, in accordance with the same principle as 

 the scientific apparatus of Pixii and Clarke. It produces alternate currents, 

 and, as it was in the first instance destined for the decomposition of water or 

 for electro metallurgy, it was provided with a commutator for bringing the 

 currents into one constant direction. When the question was raised of apply- 

 ing it to the production of light, M. Van Malderen, who had then become the 

 mechanical engineer to the Compagnie 1' Alliance, conceived the happy idea of 

 suppressing the commutator, which is difficult to maintain, and has the effect 

 of more or less weakening the current. The luminous intensity was found to 

 be appreciably augmented, and the fact was soon acknowledged that alternate 

 currents are, cceteris paribus, more favourable regulators than those in a con- 

 stant direction. The machines of the Compagnie 1'Alliance had originally 

 six discs ; these were reduced to four when the improvements introduced into 

 the coils and the magnets permitted of a greater intensity being obtained with 

 these smaller machines than with the former. In the case of lighthouses, 

 where there cannot be too great intensity, the number of six discs has been 

 preserved. 



The central depot in Paris has retained, since 1860, the first specimen con- 

 structed by M. Van Malderen of this machine, with the currents not brought 

 into one constant direction. It has six discs, and carries 56 magnets ; it is 

 1-63 metre high, and 1'43 metre in diameter; it gives less light than the 

 present machines, but it works very well still, and serves for the experiments 

 that are made at the depot. 



This first machine of the Compagnie P Alliance may be regarded as the 



starting point of all the attempts which have since been made of economically 



transforming power into electricity, and consequently into light. On that 



' account it is no more than right, although the machine is not included in the 



Exhibition, to make mention of it in the catalogue. 



22O3. The Original Model of the Eddystone Light- 

 house. 



The Eddystone Rocks, so named from the great variety of sets 

 of tides and currents which surround them, are situated about 14 

 miles S.S.W. of the port of Plymouth, the sea being fully 

 30 fathoms in depth. A lighthouse was constructed on these 

 rocks by Winstanley in 1696, and destroyed by a storm in 1703. 

 A second was built by Rudyerd in 1709, and was totally consumed 

 by fire in 1755. The present lighthouse was commenced in 1756, 



