Owing to the methods employed in the latest barometric determinations, the resulting altitudes are of a much higher order of accuracy than could be hoped for in previous measurements of the kind; in fact, it is believed that they are not so very far behind trigonometric levels in precision. This increased accuracy has been rendered possible by the collaboration of the Egyptian Meteorological Service, which, thanks to the abundant weather-data now received and dealt with by it, is in a position to furnish a close approximation to the sea-level pressure at any point in the deserts at any given instant. The aneroids used in the desert explorations are compared with the standard barometer of the Meteorological Service immediately before the start and immediately after the return of each expedition. At each observation in the desert, the date and time, and the temperature of the air, are noted, as well as the approximate latitude and longitude of the place. On the return of the expedition, the recorded reading is corrected for the error of the aneroid on the standard barometer, and the Meteorological Service is asked to supply the sea-level pressure at the particular place and time of the observation. From the difference of the two pressures and the air temperature, the height of the place above sea-level is then worked out directly with the aid of Jordan’s table of “Barometrische Höhenstufen.” If the place of observation lies considerably above sea-level, a small correction is applied to the observed air-temperature to allow for the temperature-gradient in the air; for of course it is the mean temperature of the air column between the place and sea-level which should be employed in the reduction. It is unlikely that with this procedure either the sea-level pressures or those observed at the places can be in error by much more than half a millimetre; and consequently, even with some slight uncertainties in the air-temperatures, the resulting altitudes may be expected to be accurate to within about 15 metres when only a single observation has been made, and to within much less than this when, as is usually the case at important points, the altitudes have been calculated from a series of readings spread over several days. That this degree of accuracy is actually attained seems evident on comparison of the levels obtained for the same places on different dates. Thus, for instance, at a camp close to Bir Terfawi, during a stay of five days in January and two days in March 1925, I made in all eighteen aneroid observations. The mean altitude found was 244 metres; the average deviation of a single observation from this mean was 8 metres, the maximum being 16. On the assumption that all sources of error are accidental, the probable error of the mean resulting altitude works out at less than 2 metres, which is about the same as might be expected in trigonometric levelling to the place from the Nile Valley.