Abstract We have now three investigations which profess to supply information about the escape of gases from atmospheres. Two of them, those of Messrs. Cook and Bryan, reason forward by the help of the kinetic theory of gas from the supposed causes; the third, which is that preferred by the present writer, reasons backward by the help of the same theory from the observed effects. Mr. Cook’s investigation, which will be found in the ‘Astrophysical Journal’ for January, 1900, seeks to compute the proportion of molecules which can attain the speed requisite for escape by means of the formula which Maxwell published in 1860, assigning the proportion of articles whose speed lies between v and v + dv in a system of colliding articles intended to represent an isotropic portion of gas.