312 Meteorological Observations at the Kew Observatory. 



which can exist in the air in the condition of vapour at the 

 temperature recorded in the second column, the number entered 

 under ' ' relative humidity" shows the proportion of vapour pre- 

 sent on the day of observation. When fog prevails the air is 

 saturated with moisture, and this number becomes l'OO. The 

 tension of vapour is the quantity by which the elastic force of 

 air (represented by the pressure of the barometer) at the place 

 of observation would be diminished by extracting all the aqueous 

 vapour from the air without altering its temperature. 



In the eighth column the number 10 denotes that the sky is 

 entirely clouded. The daily range of temperature (11th column) 

 is the difference between the numbers in the two columns 

 preceding. 



It must be remarked that though the reduction of the ob- 

 servations of the barometer and dry and wet bulb thermometers, 

 by means of Mr. Glaisher's tables, will generally lead to an 

 approximate mean value for the day, and to a result very near 

 the truth when several successive days are grouped together ; 

 yet, while it is the only method open to us, it is strictly appli- 

 cable only to averages extending over a period of a month, as is 

 implied by the headings of the tables of corrections, inasmuch as 

 these are derived from monthly groups of observations for several 

 successive years. On this account the corrected numbers will 

 occasionally present an anomalous appearance, which in the 

 case of temperature will be evident in the tables of observa- 

 tions, the calculated mean temperature exceeding the maximum, 

 or falling below the minimum of the day, or the reduced mean 

 temperature of the dew-point exceeding that of the air. On such 

 days the variations of temperature do not possess the general cha- 

 racter due to the season in which the observations are made. Ne- 

 vertheless, as in the calculation of the monthly means, these days 

 should have equal weight with others, it has not been thought 

 desirable to omit the reduced daily means on these occasions. 



Sourly movement of the wind. — These numbers are obtained 

 from tabulations of the anemometer registers. The mean daily 

 variation of the velocity of the wind (without regard to direction) 

 is shown by the numbers in the last column for each month, 

 from which it appears that the strength of the wind is greatest 

 about noon, and least about midnight ; and that between the 

 hours 10 a.m. and 4 p.m., its velocity is decidedly above the 

 mean, and below it between 11 p.m. and 7 a.m. The total move- 

 ment for each day is given at the foot of the several columns. 



As fer/ observatories possess the means of supplying informa- 

 tion on this subject derived from the records of an instrument of 

 such delicacy and reliability, it is thought that these results are of 

 sufficient interest to justify a more extended report of them than 

 is given of the more ordinary meteorological elements. 



