press the temperature in terms of such a form as will produce, in the refraction, independent parts that decrease rapidly. By this means he proceeds in the analytical investigation of the problem in its more comprehensive form, and deduces two equations on which its solu- tion depends.
The first of these contains the law according to which the heat decreases as the height above the earth's surface increases ; and the second determines the perpendicular ascent, when the difference of the pressures and of the temperatures at its upper and lower extre- mity have been found. If the latter, with a slight transformation, be multiplied by the proper factor, representing the variable force of gra\aty in different latitudes, it becomes identical with the usual barometric formula, all its minutest corrections included; and it has this advantage ; that, whereas the usual formula is investigated on the arbitrary assumption, that the temperature is constant at all the points of an elevation, and equal to the mean of the temperatures at the two extremities, this formula is strictly deduced from the gene- ral properties of an atmosphere in equilibrium.
Ha\ing determined, from experimental results, the values of cer- tain constants in these formulae, — first, in an atmosphere of dry air, and, secondly, in an atmosphere of air mixed with aqueous vapour, the author remarks, that the analytical theory agrees in every re- spect with the real properties of the atmosphere, as far as these have been ascertained.
The object of Mr. Ivory's further investigation is to show, that the same theory represents the astronomical refractions with a fidelity that can be deemed imperfect only as far as the values of particular constants, which can only be determined by experiment, pje liable to the charge of inaccuracy. He therefore proceeds to determine, from the formulae previously deduced, the refraction of a star in terms of its apparent zenith distance. For this purpose, the differential equations are transformed by the introduction of new symbols ; the limits of certain terms are determined previously to their being neglected ; and the equation is finally reduced to a form, in which the remaining operations consist in investigating the inte- grals of four expressions, and in subsequently assigning their nume- rical values. Great skill is displayed in conducting these intricate investigations ; and after going through the most laborious calcula- tions and computations, the author exhibits a table of theoretical re- fractions, deduced solely from the phenomena of the atmosphere, for zenith distances, extending from lO'^ to 89 These refractions are compared with those in Bessel's table, in the Tabula Regiomon- tance, and also with those in the table in the Connaissance des Temps, From this comparison, it appears, that the three tables agree within less than 1", as far as 80° from the zenith: from 80° to 88° of zenith distance, the numbers in the French table exceed those in Bessel's, the excess being 2" at 84°, and 4" at 88° ; and with a single excep- tion at 88°, (probably, judging from the character of the adjacent number, arising from an error of computation,) the refractions in the new table are nearer to Bessel's than those in the French table ;
