Page:Madras Journal of Literature and Science, series 1, volume 6 (1837).djvu/184

162 comparison not establishing the zero-point of Wrench's instrument—the other by Robinson not having been compared with any standard. The latter, however, being one of the syphon kind, may be considered free from most sources of error to which the other is liable, except those arising from the sp. gr. of the mercury, and any error in graduation, and those are not likely to exist in an instrument carefully made by to first-rate an artist. This comparison showed a difference of .007; Wrench's being in excess of the other by that quantity, before any allowance was made for capillarity—with the addition of .075 on this account, the difference was .082. This difference was not attended to in the March observations, nor in these; the observations being left as they are, for a future certain decision of the true zero-point. The correction for temperature is made as before and the same constant .075 is added for capillarity.

The thermometer is the same standard one by Troughton as was used in March, the moistened thermometer is also by the same maker, but indicating 1° higher previous to applying water to the bulb; this has been allowed for in the column of depressions. The dew points are calculated from Professor Apjohn's formula $$f'' =f'- \frac{\text{d}}{87}\times \frac{\text{P}}{30}$$ disregarding the correction ($P⁄30$) for pressure—which in this case is insensible.

The direction of the wind is shewn by an anemoscope, and the numbers in the eighth column express by estimation its velocity in miles per hour.

The solar radiation—or the direct heating power of the sun's rays—is measured by a Herschel's actinometer (an account of which is published in the third report of the British Association) and, as affording an interesting comparison of the intensity of the solar rays at the four opposite seasons of the year in this climate, the observations with this instrument will be continued.

The time is the true mean time of the Trevandrum Observatory, in latitude 8° 30′ 35″ north, and longitude 5h. 7′ 59″ east.

The elevation of the situation in which the March observations were made was by mistake in copying, stated to be about 50 feet instead of about 150 feet above the sea—the true elevation at which they were made has since been found to be 138¼ feet. The instruments are now placed in a building erected expressly for the purpose, in which the radiation and reflection of heat, and every other disturbing cause, are carefully guarded against. It was in this building that the present observations were made, and the cistern of the barometer is ascertained by actual levelling to be 177¾ feet above the level of the sea (distant in a direct line about two miles). The heights of the mercurial column given in the register are therefore those due to this elevation, corrected for temperature and the bore of the tube, but liable to a future correction (of some small amount) on account of the zero-point.