A primary standard barometer covering all pressures in the range 0 to 1200 mb has been designed and constructed at the National Physical Laboratory. A feature of the instrument is the use of optical probes as fiducial indicators which assess, from overhead, the positions of 11-cm diameter mercury surfaces in the barometer U-tube, thus permitting the mercury columns to be fully water-jacketed. The two probes use a single mercury pool as a common reference surface and their movements are assessed in terms of readings on associated line standards of Invar by means of fixed micrometer microscopes. The U-tube assembly is supported on an anti-vibration mount. The accuracy of height measurement of barometric columns was estimated by reference to measurements on end-gauges mounted in the empty U-tube and found to be better than 0.003 mm (standard deviation). Taking into account the small uncertainties in mercury density and local gravity the overall accuracy of a single pressure measurement is estimated to be about ±0.01 mb on a 99% certainty basis. At about 1000 mb, check comparisons with N.P.L. primary barometers of earlier design support this assessment.