celebrated and important experiment of Torricelli, the attention of some of the philosophers has been drawn, in succession, to this interesting problem; and the difficulty of estimating the quantity and effects of aqueous vapour, has hitherto been one of the chief obstacles to the attainment of accuracy. Daniell's hygrometer is, perhaps, better calculated than any other instrument, to remove this obstacle. This hygrometer was used in the measurement of the height of the Worcestershire Beacon, which has been deduced according to the formulae of Maskelyne, Hutton, and Daniell, from the following preliminary observations made by myself.[1]
- ↑ These formulas will be found detailed in the first volume of the Analyst, p. 396; in the Annals os Philosophy, vol. 13, p. 302; and in Daniell's Meteorological Essays, p. 188. We subjoin the directions given by the latter author, and their application.
1. Observe the heights of the barometer at the top and bottom of any elevation, and the heights of the attached thermometers. Observe, also, the temperature of the air in the shade at the two stations, by a detached thermometer; the dew-point must also be accurately taken at the upper and lower situation: these observations being made as nearly as possible at the same time.
2. Reduce the heights of the barometer observed at the top and bottom of the station, for the expansion of mercury and the mean dilatation of the tube (found by the table) at the temperatures observed by means of the attached thermometer, at the two stations.
3. Take the difference of the common logarithms of the two heights of the barometer, (so corrected) considering the first four figures as whole numbers, which will give an approximate height in fathoms.
4. Find the mean of the two temperatures observed by the detached thermometer, in the shade, at the upper and lower station, and (referring to the table) note the expansion of air due to this mean temperature.
5. Find the expansion of air for vapour at these two observed points, (by the table) and taking the mean, add it to the expansion due for temperature.
6. Then as 1.0000 plus the total expansion for temperature and vapour thus found, is to 1.0000, so is 1.0000 to the specific gravity of the air corrected for the expansion of temperature and vapour.
7. Find (by the table) the increase of density in the air for the two observed dew-points, and taking the mean, add it to the specific gravity of the air corrected for expansion, which will give the correct specific gravity of the air.
