certain new forms of Thermo-Barometers. 7 
The scales are graduated as follows :—The scale er is formed 
in exactly the same way as the thermometrical scale of the in- 
strument before described,—with this difference, that the gra- 
duations in this case are made when the tube S P is placed at an 
angle of 45° to the horizon. The globe being placed in the 
frame with its tube inclined at an angle of 45°, these marks of 
temperature are transferred to the blade GK of the shding 
square and numbered accordingly, thus forming the thermome- 
trical scale mn. The specific gravity of mercury being taken at 
7-5 times that of the sulphuric acid, a unit of 7°5 inches is taken 
on each side of the central point p of the sliding scale DC, 
and divided into 100 equal parts; then each of those parts, 
or ‘075 of an inch, will read ‘01 of an inch of mercury ; and if 
29:5 be the mean pressure at which the scale er is made, the 
point p will be numbered 29:5; and the point coinciding with 
fifty of these divisions below the point p will correspond to 30 
inches of mercury, and so on. 
The indications of this instrument are independent of the 
volume of the globe, as well as of the section of the tube: the 
a Ve 
ratio — only affects the range of the scales of temperature er 
and mn, which are determined by direct experiment. The su- 
periority of this instrument in point of accuracy over the one 
previously described, depends on this circumstance, as well as 
upon the great extent of its range. 
It will be readily seen that the level of the liquid in the globe 
is not at all affected by any change of position. The adjust- 
ments for any pressure and temperature of the air being made, 
it is obvious that the gas in the globe, having the same volume, 
must also have the same pressure that it had at the same tempe- 
rature when the tube was in its normal position ; hence it follows 
that the difference in the vertical column of liquid must exactly 
indicate the change that has taken place in the pressure of the 
atmosphere. Thus let /, be the vertical column of liquid at ¢, 
temperature, and P, pressure of gas in the globe when the tube 
was in its normal position, that is, when the atmospheric pres- 
sure was p; and let h, be the vertical column at the same tem- 
perature ¢, when the pressure of the atmosphere is pz ; then, as 
the elastic force of the gas is the same in both cases, we have 
pt+ Suv and also po+ =p, 
h h 1 
“. pt =| =Pot re WP sneea Bp (hg—hy) 5 
that is, the difference of atmospheric pressure is exactly propor- 
tional to the difference between the vertical columns. 
