A network of cliff dwellings cut by prehistoric Amerindians and then abandoned without so much as a how-do-you-do confounded archaeologists at the turn of the century. But one man had an idea: Maybe climatic change had something to do with the exodus. And so dendrochronology, or the science of dating events and climatic changes by studying growth rings in trees. When machinist Alan Lane from Velmex Inc. of Bloomfield, N.Y., got a call from Paul Krusic at the Lamont-Doherty Geological Observatory, modern photonics joined the effort.Krusic needed an operator-efficient measurement system with resolution in microns that offered computer interfacing capability and portability at a reasonable cost. Lane built an answer: the Velmex TA system. To a Velmex UniSlide series TA4000 H1 Rapid Advance, Lane added a linear encoder from ACU-RITE Inc. of Jamestown, N.Y., and a QC-1000-MAR readout from Metronics of Bedford, N.H. The UniSlide TA4000H Rapid Advance allowed Krusic to position the device with a 10 x 1-mm lead screw for incremental positioning or to disengage the lead screw and rapidly reposition the slider to a new location. This unique feature is ergonomically desirable.The ACU-RITE linear encoder offers researchers equipment with a variety of resolutions (between 0.01 and 0.001 mm). For Krusic and other dendrochronologists, resolution is an essential component to studying low-growth trees in the desert, the Arctic or other difficult terrain, where growth rings are closely packed. The linear encoder uses a sensitive photodetector to read light and dark patterns caused by shining an infrared light through a glass plate with chrome ticks. This is far more accurate than a rotary encoder that must rely on the accuracy of the lead screw, which can be affected by contaminants, temperature and other factors.After further enhancements by Velmex, the TA system offers various "travel distances" to accommodate core samples ranging from giant sequoias to oaks -- or even taking samples of sea-borne mollusk shells, a separate application. While the TA system does come with a self-contained power pack adaptable to both US and European standards and free tree-ring reading software, it does not include the PC or Mac computer, microscope or video camera recording equipment.So consider as you throw another log on the fire: That tree could reveal secrets of the past if someone would only take the time to count its rings.