Mechanical Science. 3G7 



Art. XIV.—MISCELLANEOUS INTELLIGENCE. 



I. Mechanical Science. 



1. Bridge at Menai Straits. — The first great iron plate for 

 forming; the fastening of Menai bridge was laid in its proper po 

 sition in the bottom of one of the caverns which had been 

 formed out of the solid rock on the Anglesea shore, on Easter 

 Monday. Sir Henry Parnell and Mr. Telford attended on the 

 occasion, and did not leave unfil all the necessary arrange- 

 ments were adopted for proceeding immediately with the 

 putting up of the large quantities of the iron-work which have 

 arrived from Shropshire, for form.ing the suspending cables. 

 Nearly the whole of the bridge masonry is completed, the 

 pyramids for supporting the cables of 50 feet in height above 

 the top of the main piers will be finished early in summer, and 

 the iron-work is going on so rapidly at Mr. Hazeldine's forges, 

 that there is a certainty of this great work being completed in 

 the most satisfactory manner for the use of the public, in little 

 more than another year. 



2. Gas Lighting. — The length of streets already lighted in 

 this metropolis with gas is 215 miles ! and the three principal 

 companies light 39,504 public lamps, and consume annually 

 about 33,158 chaldrons of coals, 



3. Artificial Formation of Haloes. — The following experi- 

 ment, which illustrates in a pleasing manner the actual forma- 

 tion of haloes, has been given by Dr. Brewster. Take a 

 saturated solution of alum, and having spread a few drops of 

 it over a plate of glass, it will rapidly crystaUize in small flat 

 octoedrons scarcely visible to the eye. When the plate is held 

 between the observer and the sun or a candle, v/ith the eye 

 very close to the smooth side of the glass plate, there will be 

 seen three beautiful haloes of light at diflPerent distances from 

 the luminous body. The innermost halo, which is the whitest, is 

 formed by the images refracted by a pair of faces of the 

 octoiidral crystals, not much inclined to each other ; the 

 second halo, which is more coloured, with the blue rays out- 

 wards, is formed by a pair of faces more inclined ; and the 

 third halo, which is very large and highly-coloured, is formed 

 by a still more inclined pair of faces. Each separate crystal 

 forms three images of the luminous body placed at points 120° 

 distant from each other in all the three haloes ; and, as the 

 numerous small crystals have their refracting faces turned in 

 every possible direction, the whole circumference of the haloes 

 will be completely filled up. 



The same effects may be obtained with other crystals, and 

 when they have the property of double refraction, each halo 

 Vol. XV. 2 B 



