296 Captain Sabine's comparison of barometrical and 
both cases, when the height of the mercury in the tube was 
30,400 inches, which thus became in both barometers a neu- 
tral point at which no correction was needed, equivalent to 
the more ordinary adjustment of the zero of the scale to the 
level of the cistern ; and from whence, at all other heights of 
the mercury, a correction would obtain, additive to the reading 
if the height should exceed 30,400 inches, and subtractive if 
below that amount : the diameters of the tubes were respec- 
tively .31 and .15 inches; and their ratio to the capacity of 
the cisterns being of Jones’s as 1 to 11, and of Newman’s as 
1 to 54, tt> and yt respective differences between the 
height of the mercury indicated by the scale, and 30,400 in- 
ches were the corrections to be applied. 
The temperature of the mercury at the time of observation 
was shown in both the barometers by a thermometer having 
its bulb in the cistern and immersed in the mercury itself ; 
the verniers were furnished with a tangent screw for slow 
motion, and being made to encompass the tube, assured the 
proper position of the eye in adjusting the zero of the vernier 
to the surface of the mercury ; the adjustment was capable 
of being made with much precision by the assistance of a 
microscope. 
The instruments having been thus constructed indepen- 
dently of each other, and without reference to other baro- 
meters, were compared at Mr. Newman’s house, on their 
completion in May 1823, by Mr. Daniell, Mr. Newman,* 
Mr. Jones, and myself, as follows : — 
