Which way, captain?
In the novel Moby Dick, the wooden whaling ship meets a typhoon south-east of Japan and is subjected to thunder, lightning and displays of St Elmo鈥檚 fire. Subsequently, the magnetism of the ship鈥檚 compass needle is discovered to be reversed. Author Herman Melville maintained that such compass reversals 鈥渉ave in more than one case occurred to ships in violent storms鈥�, and sometimes when the rigging has been struck by lightning the magnetism in a compass needle may be totally lost. Is this fact or fiction, and if true how does it occur?
鈥� Herman Melville鈥檚 assertion is entirely plausible. Lightning involves very high currents with high associated magnetic fields. They remagnetise exposed outcrops of high-coercivity (high resistance to the effect of an applied magnetic field) rocks with ease. Currents exceeding 10,000 amperes have been deduced from rock magnetisations. The associated magnetic fields could easily demagnetise or reversely magnetise a compass needle.
Alan Reid
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Leeds, UK
鈥� A moving electrical field will induce a magnetic field, and an electrical discharge such as lightning can easily cause a compass needle to lose or reverse its magnetisation.
What the questioner failed to mention is that Captain Ahab fashioned a new compass by striking a sailmaker鈥檚 needle and thereby magnetising it. This is rooted in fact. I have demonstrated this phenomenon more than once by accidentally dropping a pair of expensive tweezers used for handling metallurgical samples. The fall was sufficient to magnetise the tweezers.
Ferromagnetic materials are composed of microscopic magnetic domains, which may be oriented in random directions producing a demagnetised state. By aligning the domains in more or less the same direction, the material becomes magnetised. In some cases, a sharp blow will impart enough energy to make this happen.
Roger Ristau
Institute of Materials Science
University of Connecticut, Storrs, US
Hidden depths
I always believed that the sea looked blue because it reflected the colour of the sky. On holiday in Malta the sea was a very clear, deep azure blue inside caves where there was no reflected sky. What caused this colour?
鈥� Seawater appears blue because it is a very good absorber of all wavelengths of light, except for the shorter blue wavelengths, which are scattered effectively. The light attenuation is caused by the combined absorption and scattering properties of everything in the water, along with the water itself.
Changes in the sea鈥檚 colour are primarily due to changes in the type and concentration of plankton. Tropical oceans are clear because they are lacking in suspended sediment and plankton, which contrasts with the popular misconception that tropical waters have a high biological productivity. In fact, they are virtually sterile compared with the cooler, plankton-rich temperate ocean regions. Inorganic particulates and dissolved matter also reflect and absorb light, which affects the clarity of the water.
Johan Uys
Bellville, South Africa
鈥� The effect is caused by the selective absorption of light by water molecules, chiefly the oxygen component, that take out the red end of the visible light spectrum. In a similar way, ice masses at the poles and big icebergs look blue.
Albert Day
Norfolk, UK
鈥� Reflection of light contributes to the colour of the open sea, but does not determine it. Even pure water is slightly blue-green, because it filters out the red and orange content of light. However, impurities in seawater, especially organic substances, affect its appearance far more drastically.
In caves like those described, the light coming in must travel through a greater thickness of seawater than the light we usually see. The strong absorption of wavelengths other than blue and green intensifies the ethereal effect. In fact such light contains so little red that navy personnel who have been on submarine duty for several days find everything looks unnaturally ruddy when they return to the surface.
Jon Richfield
Somerset West, South Africa
鈥� Blue Lake near Mount Gambier in South Australia is always blue, sun or no sun. The lake is situated in a limestone area and is saturated with calcium carbonate. The colour comes from the greater scattering of blue light by very fine particles of the compound suspended in the water.
Seawater is normally supersaturated with calcium carbonate, but magnesium in the water tends to stop it precipitating out. However, this can occur when seawater comes into contact with the calcium carbonate mineral calcite in rock or soil. It is possible this what is happening in the caves of Malta.
Robert Gerritse
Wembley Downs, Western Australia
This week鈥檚 question
Cloud cover
A group of cumulus clouds was passing over my house in Tuscany, Italy, on a September afternoon (see Photo) lit by the low sun at about 5 pm. A clear shadow of the lower clouds was appearing on the undersurface of the upper clouds. Given that the light came from above both sets of clouds, how was the shadow projected on the higher ones?
Alessandro Saragosa
Terranuova, Italy