Lowdown on Earth’s heat
“Science Stats” (SN: 8/27/11, p. 4) understates the power Earth radiates into space and mistakenly suggests that Earth radiates more energy from internal sources than it receives from the sun. The total (44 trillion watts) shown in your diagram must represent only the minuscule percentage (about 0.02 percent) from internal energy sources (radioactivity, tidal, remnant gravitational energy) that cause surface effects like volcanoes and plate tectonics. In fact, Earth receives about 174,000 tera­watts from the sun and radiates the same amount. Strictly speaking, Earth doesn’t radiate any “heat.” Instead, it emits energy in the form of light, predominantly at infrared wavelengths because of its modest temperature.
Don McCarthy, Tucson, Ariz.

The reader is correct. The diagram presents heat flow only from Earth’s internal energy sources. — Camille M. Carlisle

Zapped on units
The article “Dolphin can sense electric fields” (SN: 8/27/11, p. 12) contains an error in the sentence, “Paco could perceive a current as weak as 4.6 microvolts per centimeter….” Current is measured in amperes, whereas electric field is measured in volts per meter.
Robert P. Yassanye, Sarasota, Fla.

The reader is correct. Paco perceived an electric field, as the reported units indicate, not current. — Nadia Drake