Seismic waves are studied by seismologists and measured by a seismograph or seismometer. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer. The Alaska Volcano Observatory completed installation of another seismometer near Fourpeaked. Unlike the continually recording seismometer, accelerometers nearly always work in a triggered mode. The basic damped horizontal pendulum seismometer swings like the gate of a fence. The Volcanological Observatory there did not get its first seismometer until some three years later. The tiny transmitter in the seismometer capsule continued sending out a 50-milliwatt signal. The seismometer penetrator design for LUNAR-A may be reused in a future mission. The "Viking 2" seismometer did detect vibrations from Mars winds complementing the meteorology results. A strong-motion seismometer measures acceleration. Each ring consists of an odd number of seismometer sites. Another type of seismometer is a digital strong-motion seismometer, or accelerograph. Equipment indirectly related to the detector such as a seismometer were deployed in 2005. Tiltmeters have a long history, somewhat parallel to the history of the seismometer. See Zhang's seismometer for further details.