Powerful 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 US and the USSR, aeronautical engineers in the US Department of Defence 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 (1,860 miles); and intermediate up to 5,500km (3,410 miles).

Intercontinental ballistic missiles (ICBMs) travel more than 5,500km, a distance based on the shortest distance between the north-eastern border of continental US and the north-western 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 US, Russia, China, India, France, Israel and North Korea. Earlier this year, North Korean leader Kim Jong-un committed to denuclearisation.

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 – which is also known as Tower 42 – rise from a hole in the ground to a majestic 183-metre-high building.

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