550 PROFESSOR DANIELL ON THE WATER-BAROMETER ERECTED 
but generally at 9 a.m. and 3 p.m. They include a very considerable range of 
temperature (from 57° to 74°), and serve to test the accuracy of the instruments 
brought into comparison shortly after the completion of the water-barometer, 
and that of the different corrections which have been applied to them. 
The first column of the following Table records the date, and the second 
the hour of the observations. The third column contains the temperature of 
the internal thermometer (c, d), and the fourth that of the external thermo- 
meter (6,7)- The fifth shows the corrected height of the water-barometer; 
the sixth the temperature of the thermometer attached to the mercurial barome- 
ter. This, it will be observed, sometimes differs several degrees from the former; 
and, when this is the case, the mean has been taken as the temperature by which 
to correct the length of the scale ; as standing at the bottom of the column, it 
most probably indicated the temperature of the lower extremity. The seventh 
column contains the corrected height of the mercurial barometer. In the eighth 
column I have placed the height of the column of water reduced to the corre- 
sponding height in mercury. As the basis of this calculation, I have taken the 
specific gravity of mercury at 40°, 13*624, as determined, at my request, by 
Mr. Faraday at the time when I fitted up the large mercurial barometer be- 
longing to the Society. The ninth column exhibits the differences of the two 
columns, or the amount of the depression of the column of water by the in- 
cluded vapour, expressed in parts of an inch of mercury. 
By the side of these differences I have placed, in the tenth column, the elas- 
ticity of aqueous vapour due to the temperature of the surface water in the 
barometer, calculated from the data of Dr. Ure. The eleventh column exhi- 
bits the differences of the two preceding. The mean results of every ten ob- 
servations are also added to the register. 
