194 FRAGMENTS OF SCIENCE. 



In a first-class " dioptric " apparatus the light emanates 

 from a lamp with several concentric wicks, the flame of 

 which, being kindled by a very active draught, attains to 

 great intensity. In fixed lights the lenses refract the rays 

 issuing from the lamp so as to cause them to form a lumi- 

 nous sheet which grazes the sea-horizon. In revolving lights 

 the lenses gather up the rays into distinct beams, resem- 

 bling the spokes of a wheel, which sweep over the sea and 

 strike the eye of the mariner in succession. 



It is not for clear weather that the greatest strength- 

 ening of the light is intended, for here it is not needed. 

 Nor is it for densely foggy weather, for here it is in- 

 effectual. But it is for the intermediate stages of hazy, 

 snowy, or rainy weather, in which a powerful light can 

 assert itself, while a feeble one is extinguished. The 

 usual first-order lamp is one of four wicks, but Mr. Doug- 

 lass, the able and indefatigable engineer of the Trinity 

 House, has recently raised the number of the wicks to six, 

 which produce a very noble flame. To Mr. Wigham, of 

 Dublin, we are indebted for the successful application of 

 gas to lighthouse illumination. In some lighthouses his 

 power varies from 28 jets to 108 jets, while in the light- 

 house of Galley Head three burners of the largest size can 

 be employed, the maximum number of jets being 324. 

 These larger powers are invoked only in case of fog, the 

 2 8- jet burner being amply sufficient for clear weather. 

 The passage from the small burner to the large, and from 

 the large burner to the small, is made with ease, rapidity, 

 and certainty. This employment of gas is indigenous to 

 Ireland, and the Board of Trade has exercised a wise 

 liberality in allowing every facility to Mr. Wigham for the 

 development of his invention. 



The last great agent employed in lighthouse illumination 

 is electricity. It was in this Institution, beginning in 

 1831, that Faraday proved the existence and illustrated 

 the laws of those induced currents which in our day have 

 received such astounding development. In relation to 

 this subject Faraday's words have a prophetic ring. " I 

 have rather," he writes in 1831, "been desirous of dis- 

 covering new facts and new relations dependent on mag- 

 neto-electric induction than of exalting the force of those 

 already obtained, being assured that the latter would find 

 their full development hereafter/' The labors of Holmes, 



