The value of the following meteorological observations depending on the goodness of my instruments, on certain precautions in the use of them, and on the care with which atmospheric changes were recorded, I shall preface my notices on the meteorology of Dukhun with an account of the instruments I had in use, and of my method to insure correct results. In determining atmospheric pressure, for the first two years I was confined to two of Thomas Jones’s barometers: they required to be filled when employed, and were destitute of an adjustment for the change of level of the mercury in their cisterns, unless the position of the cistern had been altered at each observation; a measure attended with insuperable inconvenience. At first I experienced a good deal of vexation in expelling the moisture from the tubes; but by previously rubbing the inside with a tuft of floss silk tied to the end of an iron wire, I dried them so effectually (unless in the monsoon months) as to excite powerful electricity: and I have frequently had shocks in my right thumb, running up to my shoulder, in pouring the mercury into the tube, accompanied with cracking noises, until the approach of the mercury to within two inches of my thumb, when the electricity was discharged as described. I experienced these shocks at Salseh, near Purranda, on the 3rd of February; at Pairgaon, on the Beema River, on the 14th of February; at Kundallah, in the hilly tracts, on the 14th of March, 1828; and at many other places. Jones’s barometers were each provided with a thermometer let into one of the legs of the tripod on which the barometer was suspended. The scale of this thermometer was of thin ivory, and the tube excessively slender. During the heat of the day in the dry season, the scale was contracted, by parting with its moisture, into the segment of a circle, bending the tube of the thermometer. At night the ivory scale relaxed from its curvature, and at sunrise it had returned to a right line. This operation continued daily for more than three weeks ; but on the 15th of February 1827, the contraction of the scale was too great for the flexibility of the glass, and the tube of thermometer No. 1. broke. The thermometer attached to barometer No. 2. subsequently shared the same fate, from a similar cause. Thomas Jones’s barometers pack well, carry easily, and are certainly very useful as checks upon permanently filled barometers, which frequently give false indications, from the unknown escape of the mercury, or the admission of air, which could not be detected without the aid of a second barometer: but they are very troublesome to fill; are destitute of a thermometer near the cistern, to determine the temperature of the mercury; and want the means of adjusting the lower level of the barometric column; the tubes are frequently breaking, from the pressure of the iron screw which fixes the cistern to the tube, (I have broken seven tubes from this cause,) and in case of not being tightly screwed on, the cistern falls off from the weight of the mercury in it, and the mercury is lost; and from the uncertainty of expelling air and moisture from the tubes, particularly in the moist months, the indications of the instrument can only be looked upon as approximations to the truth. On the 12th of April 1827, I had the gratification to receive three barometers from England : they were made by Cary on the Englefield construction, which admits of a most delicate adjustment of the lower level of the barometric column in the reservoir. They were beautifully finished, but unluckily had reservoirs of ivory ; and I instantly foresaw the inconvenience to which such selection of material would subject me. In the dry weather the ivory contracted, and permitted the escape of the mercury by the screws (male and female) which joined the two portions of the reservoir. Subsequently the reservoirs cracked at the spots where the metallic screws attached the reservoir to the brass cylinder surrounding the tube of the barometer. I was finally compelled from these disasters, within a twelve-month, to send two barometers back to England to have glass or iron reservoirs put to them. From the ease, accuracy, and delicacy with which the contrivance in these instruments permits the mercury to be adjusted at its lower level, they require only an iron cistern to render them quite efficient; and they are peculiarly suited to measure minute changes in the atmospheric tides. Mr. Newman of Regent-street has acted upon my suggestion, and has constructed two Englefield barometers with iron cisterns, to which he has applied an excellent improvement of his own to prevent the oscillation of the mercury in the tube
en route
.
Lunisolar Atmospheric Tides. II
