NOVELTY - A Herriott type multipass cell (10) comprises center axle (74), housing (80A, 80B) surrounding and spaced from (74) to provide elongated annular area (84), and pump for moving a gas through (84). A laser or light emitting diode emits light beam into (84) via entry aperture (30) in an annular mirror (44). The beam is reflected several times between (44) and another annular mirror (46) before exiting (10) via an exit aperture (48) in (46) and impinging on a photo detector (52).    USE - For detecting and measuring the concentration of gases (e.g. methane, butane, propane, ethane, oxygen, hydrogen, nitrogen, water, hydrogen fluoride, hydrogen chloride, hydrogen boride, hydrogen sulfide, ammonia, carbon monoxide, carbon dioxide, nitric oxide, nitrogen dioxide and sulfur hexafluoride) by absorption spectroscopy; and for conducting survey of a geographical area to find occurrence of gas leakage.    ADVANTAGE - The cell is highly portable, rugged, sensitive, and can be easily mounted in vehicles so that the gas can be continuously picked up from the environment and cycled through the cell while the vehicle is moving to enable an operator to expeditiously survey a large geographical area. The cell provides readouts coordinated with a global positioning monitor so that gas level intensities of a geographical area can be quickly and accurately mapped. The cell can be moved at a faster rate over a geographical area due to the unique features of an annular absorption area providing a long beam path for a small volume of gas and a pump which quickly changes the gas within the area. The cell can be moved by a moving vehicle at a speed that enables a large geographical area to be checked for possible gas leakage in a short time. The cell can also measure absorption after a single pass of the light beam.    DETAILED DESCRIPTION - The cell comprises different photo detectors for monitoring the intensity of the unattenuated light beam and for detecting the intensity of light transmitted through (46) after a single pass through (10). The light source is operatively connected to a heat control assembly having a heat sink and the gas is passed to the heat sink to augment temperature control of the laser or light emitting diode.    An INDEPENDENT CLAIM is included for a method of detecting gases.    DESCRIPTION OF DRAWING(S) - The figure shows an elevational cross sectional view of a Herriott type cell.    Herriott type multipass cell (10)    Entry aperture (30)    Annular mirrors (44, 46)    Exit aperture (48)    Photo detector (52)    Center axle (74)    Housing (80A, 80B)    Elongated annular area. (84)