162 Horary Meteorological Observations at Trevandrum. [July 



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 so 

 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 / ' = /- — ixl disre - 



garding the correction (|^) for pressure— which in this case is in- 

 sensible. 



The direction of the wind is shewn by an anemoscope, and the num- 

 bers 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 accounfof which is pub- 

 lished 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 138i 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 177| 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, correct- 

 ed for temperature and the bore of the tube, but liable to a future cor- 

 rection (of some small amount) on account of the zero-point. 



