210 J. noPKINSON METEOEOLOGICAL OBSERVATIONS 



The present locality is not quite so open as the former one, but 

 there are no buildings sufficiently near to interfere with the indi- 

 cations of the thermometers or the rain-gauge, the position and 

 exposure of which fulfil the requirements of the Meteorological 

 Society. The wind-currents may possibly be slightly affected by 

 tlie houses in the neighbourhood, and to a greater degree by the 

 form of the ground ; for the position is on a slight slope about 

 half-way between the highest ground in the neighbourhood and 

 the alluvial plain in the valley of the Colne, so that a northerly or 

 southerly wind along the valley may be more felt than an easterly 

 or westerly across it. This would affect the determination of the 

 velocity more than that of the direction of the wind. 



The longitude of the station is 0° 23' 40" W. (of Greenwich), and 

 the latitude is 51° 39' 45" N. The centre of the town of "Watford 

 is scarcely half a mile to the south. The ground level, where the 

 thermometer-screen and rain-gauge are placed, is 223 feet above 

 Ordnance Datum (mean sea-level), and the cistern of the barometer 

 is 233 feet 6 inches above this datum. 



With regard to the instruments used, detailed particulars have 

 already been given in our ' Transactions.'* The thermometers are, 

 as before, four feet above the ground, in a "Stevenson" screen 

 over grass, and the rain-gauge rim is one foot above the ground. A 

 vane has been added by which the direction of the wind is now 

 ascertained. It was constructed by Mr. Hicks, of Hatton Garden, 

 by whom all my instruments have been made, and as it rotates 

 with the slightest breath of air and has a good exposure, the direc- 

 tion of the wind will most probably be more accurately determined 

 than before. The " Snowdon " rain-gauge aloue has been used, 

 the " Howard " having been left at Holly Jiank for the continuance 

 of observations there. 



The whole of the observations are taken at 9 a.m., or sometimes 

 a few minutes before that hour. The reading of the minimum 

 thermometer is entered to the same day, and that of the maximum 

 thermometer and the rain-gauge to the previous day. All the 

 readings are corrected for the index errors of the instruments, and 

 the readings of the barometer are corrected for temperature and 

 altitude. The mean temperature is deduced from the readings of 

 the dry-bulb (9 a.m.), the maximum, and the minimum thermo- 

 meter, being the arithmetical mean of the three readings. 



The accompanying tables (pp. 212, 213) give the monthly means 

 of the daily observations, and of other results deduced from them. 



The tables have been slightly altered from those given with the 

 previous reports. The discontinuance of the 9 p.m. observations 

 necessitating the mean temperatui'e being deduced as already stated, 

 I have thought it desirable to give, in the first table, the monthly 

 mean readings of the dry-bulb thermometer at 9 a.m. They will 

 be found to vary but slightly from the adopted monthly means ; 

 and in the year the difference is only 0°-2 ; while between the 



* Vol. I, pp. 217, 218. 



