632 PROCEEDINGS OF SECTION H. 



with loose asbestos saturated with acetone, it may be safely 

 handled, and in this form is much used for buoys and beacons. Or 

 a buoy or beacon may contain a store of carbide to which water is 

 admitted, the supply being governed by the pressure of the gas in 

 the receiver. Wi k lamps burning a month or more without 

 attention are much used for similar work, the wicks being either 

 carefully carbonised in a manner that allows them to retain their 

 capillarity for oil ; or made to burn, not at the end, but at the 

 bight, where it travels slowly over a small roller in the burner, the 

 motion being produced by the end of the wick being attached to a 

 float in a cistern of oil that slowly leaks away. 



Powerful lights are necessary to maintain the visibility in 

 hazy weather, but there is no evident proportion between the 

 candle-power and the range of visibility. The latter is really 

 more affected by the height of the light above the sea level. In a 

 perfectly transparent atmosphere every light would be visible to 

 its geographical limit, but a slight haze soon renders weak lights 

 invisible. To increase the range in such an atmosphere by a given 

 factor we have to increase the candle-power by approximately 

 the tenth power of that factor ; or to double the range we must 

 increase the power a thousand times ; but the problem is not 

 capable of being solved by a mathematical formula, the data are 

 so uncertain. 



To render the lights distinctive they may be made flashing, 

 occulting, or coloured, or all or any of these in combination. 

 Coloured glasses stop much of the light, and are only proper for 

 harbour work. Occultations may be produced either by turning 

 up and down the gas or by rotating round the light a discontinuous 

 opaque screen. Flashes are produced by rotating the optical 

 apparatus itself, which then consists of spherical or bulls-eye 

 lenses surrounded by curved glass prisms, arranged as a series of 

 panels. The faster the optic revolves, the fewer the panels needed 

 to produce the sequence of flashes ; to enable the heavy mass of 

 the optic to revolve frictionlessly it is supported in modern instances 

 by being floated in a bath of mercury in a circular castiron tank, 

 seven or eight hundredweight of mercury suitably disposed buoying 

 up five tons or more of the cut-glass and gun-metal forming the 

 optic. 



Cape Byi"on light thus rotating on mercury revolves once in 

 ten seconds, while Macquarie (the South Head light) takes 16 

 minutes to go round on its chariot of rollers. But the duration of 

 the flash is much reduced by this quick speed, and Cape Byi'on only 

 shows for one-fifth of a second every five seconds. When these 

 feux eclairs, or lightning flashes, were first introduced, one-tenth 

 second was thought enough, but it was found too short for the 

 eye to fully appreciate. One-foui'th second is enough, however, 

 and if the flashes succeed each other often enough this jcu eclair 

 system is the most powerful, economical and effective system of 

 lighting, especially when the flashes are arranged in groups, as 



