METEOROLOGICAL OBSERVATIONS. 



231 



m 



closed at the upper end. This tube contains a 

 column of pure mercury, about 30 inches high., 

 inverted over (b) a cistern also containing 

 mercury. The pressure of the atmosphere on 

 the mercury in the cistern supjDorts the column 

 of mercury in the tube, and as the pressure 

 increases or diminishes, the mercury rises and 

 falls accordingly. A graduated scale, together 

 with a vernier (o) for reading, is attached to 

 the brass frame parallel to the tube. The special 

 feature of barometers constructed on Fortin's 

 principle is that the level of the mercury in the 

 cistern can always be adjusted previous to each 

 observation to a fixed zero point, consequently 

 any loss of mercury through leakage or 

 oxidation is of little importance, as it does not 

 affect the accuracy of the readings of the 

 instrument. 



The mean annual value for pressure, as 

 will be seen from the last page of the tables 

 (see also diagram 1) is 29*967 inches. The 

 maximum pressure observed, 30-937 inches, was 

 reached on January 1 8th, 1 882, and the minimum 

 pressure, 28-330 inches, occurred on December 

 29th, 1899, which gives an extreme range of 

 2 '60 7 inches. 



Corrections for Index error (+ 0-008), 

 capillarity (+ 0-013), height above mid-tide (43 

 feet), and temperature have been applied to the 

 tables. 



Temperature (Diagram 2). The mean 

 temperature of the air is of great importance 

 in determining the suitability of the climate for 

 persons having a tendency to any particular 

 disease and also for health purposes generally. 

 It is also important for hygienic reasons, that 

 the range of temperature or difference between 

 that of the day and night should not be great. 

 Cornwall is particularly fortunate in these 

 respects. At Truro the mean annual tempera- 



