THE THERMOMETER ii 



can ascertain whether the mean pressure of any clay is above 

 or below the average. 



The mean pressure of a day is, of course, independent 

 of the diurnal variation ; and it will be noticed in Table III. 

 that if we take the mean of the three reduced readings at 

 7 a.m., 3 p.m., and 11 p.m., we shall get a very close approxi- 

 mation to the mean pressure of the day as deduced from the 

 twenty-four hourly readings. 



The best hours for observation of pressure are therefore 

 7 a.m., 3 p.m., and 11 p.m., especially as it will be found in 

 the next section that the 7 a.m. and 3 p.m. readings of the 

 thermometer are the best hours for the observation of 

 temperature. 



The Thermometer 



was invented by Galileo towards the end of the sixteenth 

 century. His air thermometer consists of a glass bulb with 

 a long neck, which is dipped into a small cistern or vessel 

 containing coloured liquid. As the air in the bulb expands 

 or contracts according as its temperature increases or 

 diminishes, so the coloured liquid ialls or rises in the neck, 

 to which a scale is attached to mark the variations. But in 

 order that the coloured liquid may have a convenient normal 

 height in the neck, it is, of course, necessary to heat the 

 bulb before the neck is plunged into the liquid. 



Such an instrument as this will show variations of 

 temperature, provided that the atmospheric pressure remains 

 the same ; if the atmospheric pressure increases or diminishes, 

 the elastic glass bulb will contract or expand and vary the 

 height of the liquid. 



The instrument was afterwards improved; the neck 

 became a thin stem ; the bulb was greatly reduced in size 

 and filled with spirits of wine or mercury ; and the glass 

 stem was hermetically sealed by means of the blow-pipe, so 

 as to exclude the air entirely. Variations of temperature 

 were now shown by the expansion or contraction of the fluid ; 

 and all that was required was a scale. 



In the year 1701 Sir Isaac Newton pointed out that there 

 were two temperatures adapted for the purposes of graduation. 



