Sir, - I write in reply to Mr Bryan F. Smythe's letter (December 30th), and I thank him for his kind comments about the "Science Today" column.

To answer his specific question: the Hiroshima bomb dropped on August 6th, 1945 was a gun-assembled uranium design, and the Nagasaki bomb, dropped on August 9th 1945, was an implosion-assembled plutonium weapon.

Both the uranium and plutonium atom can undergo nuclear fission, i.e. violent rupture. When the mass of a fissionable material exceeds a certain amount, the material goes critical, i.e. an uncontrolled wave of fission explodes through the material. In the gun-assembled uranium bomb, two sub-critical masses of uranium remained separated until the bomb was fired. Then one sub-critical mass was fired into the other to make a critical mass: In the implosion-type plutonium bomb, a sub-critical mass of plutonium was surrounded by a chemical explosive. When the bomb was fired, the chemical explosive imploded on the plutonium, concentrating it into a critical density. - Yours, etc.,

University College, Cork.