692 Lord Kelvin : The Problem of 
questions connected with internal pressure throughout a 
spherical liquid mass, such as the sun. It is also useful 
when we are considering pressure and temperature in 
gaseous fluids, such as our terrestrial atmosphere, or the 
outermost parts of the sun ; which may be regarded as 
practically gaseous where the density is anything less than "1. 
§9. For a perfect gas, (8) divided by p, becomes 
<?H=S« (9). 
By this we see, what is interesting to remark, that for the 
same temperature and same gaseous material, the " height 
of the homogeneous atmosphere " is the same for the air at 
the earth's surface and for the air at any height above the 
surface ; and is the same for different barometric pressures. 
For different temperatures, it varies as the absolute tempe- 
rature. For different gases at the same temperature, it is 
proportional to their specific volumes. For different forces 
of gravity, it is inversely proportional to them. 
§ 10. Even for cosmical reckonings in respect to our 
present subject, and in many and varied terrestrial reckon- 
ings, it is convenient to take as unit of force the heaviness 
in mid -latitudes of the unit of mass. The unit of mass, for 
all nations and peoples of the earth, must for general con- 
venience be founded on the existing French Metrical System. 
The unit may, according to the particular magnitude or 
character of substance of which the mass or quantity is to 
be specified, be conveniently taken as a milligram, or a gram, 
or a kilogram, or a metric ton (one thousand kilograms), 
or 10 9 tons. 
§ 11. The choice of unit force as mean terrestrial heaviness 
of unit mass is very convenient for ordinary earthly pur- 
poses, but language in which it is adopted is, unless properly 
guarded and tacitly understood, always liable to ambiguity 
as to whether force or quantity of matter is meant. Thus 
if (using for a moment the moribund British Engineering 
reckonings in pounds, inches, etc.) we speak of 73 pounds 
of lead, there is no doubt that we mean quantity of a 
particular kind of matter ; but if we speak of 73 pounds 
per square inch (which might be 73 pounds of lead, or of 
iron, or of stone) we mean a force. If we call the pressure 
on the boiler of a ship 73 pounds per square inch, we mean 
a somewhat greater pressure when the ship is in middle or 
northern latitudes than when she is on the equator; though 
the difference is, for pressures on safety-valves, practically 
negligible, being for example three-tenths per cent, between 
the equator and the latitude of Glasgow or Edinburgh. 
