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Temperature is not measured directly but is inferred from the levels of certain isotopes (chemically identical atoms with the same number of protons but differing numbers of neutrons) of water molecules released by melting the ice cores. Water is composed of molecules comprising two atoms of hydrogen (H) and one of oxygen (O). The isotopes of particular interest for climate studies are 16O (with eight protons and eight neutrons); 18O (eight protons and 10 neutrons); 1H (with one proton and no neutrons); and 2H (one proton and one neutron, also known as deuterium, D).

Their primary site of action is in the central nervous system. Unlike local anesthetics, the general anesthetics reduce nerve transmission at the synapses, the sites at which neurotransmitters are released by neurons and adjacent nerve cells respond. General anesthetics affect the response of receptors and ion channels to neurotransmitters, thereby decreasing nerve cell activity. General anesthetics bind only very weakly to their sites of action and interact with proteins in a lipid environment, factors that together make it difficult to determine their exact binding structure.

Essentially, hackers get inside a computer system by taking advantage of software or hardware weaknesses that exist in every system. Before explaining how they do this, a few defi nitions are in order. The term “hacker” is fairly controversial: some use this word to describe those whose intrusions into computer systems push the boundaries of knowledge without causing intentional harm, whereas “crackers” want to wreak havoc. I prefer “unauthorized user” (UU) for anyone who engages in unsanctioned computer access.

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Ask the Experts (Scientific American Special Online Issue No. 25) by Scientific American

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