Meteorological Observations at the Kew Observatory. 311 



communicates its motion to a cylinder covered with paper, and 

 a pencil is driven by clockwork from end to end of the cylinder 

 in twenty-four hours ; thus a curve is traced upon the paper by 

 the combined motions of the pencil and cylinder, from which 

 we can ascertain the space passed over by the wind from one 

 moment of time to another. The hemispherical cups project 

 two feet above the dome of the observatory, which is fifty feet 

 above the ground. As an indication of the delicacy of this 

 instrument, it may be stated that it is the rarest possible occur- 

 rence to find the cups stationary even for a moment, happening 

 perhaps not oftener than twice in a year, at which time ; alone 

 the wind is too feeble to overcome the small amount of friction 

 of the axis in its bearings. 



Wind Vane. — The vane consists of a hollow parallelopepid 

 without ends, fixed to the top of a vertical rod, which is capa- 

 ble of rotating freely about its axis, and to which is attached 

 a divided circle with numbers, indicating the direction of wind. 

 The vane is two and a-half feet above the dome. 



EXPLANATION OE THE TABLES OE OBSERVATIONS. 



With the aid of the following remarks the headings of the 

 different columns will be found sufficiently intelligible. The se- 

 cond and third columns contain respectively the mean barometric 

 pressure, and mean temperature of the air, for each day, reduced 

 by means of Mr. Grlaisher's tables from the individual obser- 

 vations, and corrected for the errors of the instruments. 



The tension of vapour is calculated, from the numbers given 

 in the second and third columns, and from the corrected mean 

 readings of the wet bulb thermometer, by the following formula 

 of Dr. Apjohn : — 



t _ d_ h_ 



for reading of wet bulb thermometer above 32° ; and 



„_ , d_ h. 

 f ~J 96 X 30 



for reading of wet bulb thermometer below 32° ; f" being the 

 elasticity of vapour required ; f the elasticity corresponding to 

 the temperature of the wet bulb thermometer ; d the difference 

 between the dry and wet bulb thermometers ; and h the height 

 of the barometer. The labour of calculation has been abridged 

 by the use of a sliding rule, adapted to Apjohn^s formula, 

 invented by the late Mr. Welsh, by means of which the dew- 

 point and relative humidity were also obtained. 



By the dew-point is understood the lowest temperature at 

 which the whole of the moisture contained in the air can 

 remain in the state of vapour, any further cooling producing 

 dew. If we represent by 1*00 the greatest quantity of moisture 



