Missiles with a deadly range

QUESTION The range of an intermediate missile is 3,000 miles, so what qualifies as a long-range missile?

For the purposes of STArT (Strategic Arms reduction Treaty) — the 1991 bilateral agreement between the U.S. and the USSr — aeronautical engineers in the U.S. Department of Defense devised an arbitrary, but useful, definition for ballistic missiles.

Short-range missiles can travel less than 1,000km (620 miles); medium-range up to 3,000km (1,864 miles); and intermediate up to 5,500km (3,417 miles).

Intercontinental ballistic missiles (ICBMs) travel more than 5,500km, a figure based on the shortest distance between the north-eastern border of continental U.S. and the northwestern border of the USSr.

ICBMs are of concern to global security because they are able to deliver a nuclear payload across great distances. Countries with ICBM technology are Britain, the U.S., russia, China, India, France, Israel and North Korea.

Tim Lees, Bedford.

QUESTION How do builders ensure skyscrapers go up straight and true, and don’t lean at the top?

FUrTher to the earlier answer that described the art of skyscraper building, during my time at the Sir John Cass School of Art in Whitechapel, east London, I watched the NatWest Tower rise from a hole in the ground to a majestic 183 metres (600ft) high.

Though one of the first skyscrapers to use a laser beam to ensure it was spot-on vertically, engineers were miffed to find it was out by 7mm at the top.

Mike Shorer, Newhaven, E. Sussex.