of Edinburgh, Session 1879-80. 
433 
tube must be vertical with its closed end up, and the part of it 
occupied by the manometric water-column must be kept at a 
nearly enough definite temperature by a surrounding glass jacket- 
tube of ice-water. This glass jacket-tube is wide enough to 
allow little lumps of ice to be dropped into it from its upper end, 
which is open. By aid of an india-rubber tube connected with 
its lower end, and a little movable cistern, as shown in the draw- 
ing, the level of the water in the jacket is kept from a few inches 
above to a quarter of an inch below that of the interior manometric 
column. Thus, by dropping in lumps of ice so as always to keep 
some unmelted ice floating in the water of the jacket, it is easy to 
keep the temperature of the top of the manometric water-column 
exactly at the freezing temperature. As we shall see presently, the 
manometric water below its free surface may be at any temperature 
from freezing to 10° C. above freezing without more than per 
cent, of hydrostatic error. The temperature in the vapour-space 
above the liquid column may be either freezing or anything higher. 
It ought not to be lower than freezing, because, if it were so, vapour 
would condense as hoar frost on the glass, and evaporation from the 
top of the liquid column would either cry ophorus wise freeze the 
liquid there, or cool it below the freezing point. 
The chief object of keeping the top of the manometric column 
exactly at the freezing-point is to render perfectly definite and 
constant the steam-pressure in the space above it. 
A second object of considerable importance when the bore of the 
tube is so small as one millimetre, is to give constancy to the 
capillary tension of the surface of the water. The elevation by 
capillary attraction of ice-cold water in a tube of one millimetre 
bore is about 7 millims. The constancy of temperature provided 
by the surrounding iced water will be more than sufficient to 
prevent any perceptible error due to inequality of this effect. To 
avoid error from capillary attraction the bore of the tube ought to be 
very uniform, if it is so small as one millimetre. If it be three milli- 
metres or more, a very rough approach to uniformity would suffice. 
A third object of the iced-water jacket, and one of much more 
importance than the second, is to give accuracy to the hydrostatic 
measurement by keeping the density of the water throughout the 
long vertical branch definite and constant. But the density of water 
