From the surface to 500 fathoms the general form of the isothermals remains the same, except that instead of an equatorial maximum belt there is a focus of maximum temperature off the eastern coast of the United States.
The isothermals of mean surface temperature in the South Atlantic are in the lower latitudes of an cn- shape, temperatures being higher on the American than on the African side.
The isothermals of mean annual temperature lie over northern Asia on curves tolerably regular in their outline, having their western branches in a somewhat higher latitude than their eastern; a reduction of I° of latitude corresponds approximately - and irrespective of modifications due to elevation - to a rise of 2 ° Fahr., as far say as 30° N., where the mean temperature is about 75° Fahr.
If now we cut the freezing-point surface by planes parallel to the base ABC we get curves giving us all the alloys whose freezing-point is the same; these isothermals can be projected on to the plane of the triangle and are seen as dotted lines in fig.
(3) Expansion or compression at constant temperature, represented by curves called Isothermals, such as BC, AD, the form of which depends on the nature of the working sub stance.