648 Lieutenant Eenny on the Constants of 



I have retained in my formula the values of the coefficients of the arith- 

 metic mean of temperatures, given by detached thermometers, as recommended 

 by Laplace, for temperatures above the freezing-point ; but, as a matter of 

 course, I liave added a diminished multiplier for temperatures below the 

 freezing-point. 



In conclusion, I hope I may say, with confidence, and without presumption, 

 that the questions of the new constant, and of the action of the vapour of water 

 as affecting barometric calculations, need no further consideration. Of course, 

 if hereafter the ratio of the specific gravity of dry air to that of quicksilver be 

 more accurately determined by experiment than at present, the constant of my 

 correct formula will be thereby improved ; and, if also a more correct expres- 

 sion of the expansion of air, under increase of temperature, a corresponding 

 improvement will thereby be obtained ; but small indeed will be such improve- 

 ments compared to those which will result from the employment of correct 

 tables of horary correction, or from a more correct method than the present one 

 of estimating the mean temperature of the atmospheric column between the 

 stations of barometric observations. To these particular objects all attention 

 ought now to be directed, for here at present is the weak side of barometric 

 levelling. To obtain these objects, too many hands cannot be employed in 

 making observations, — hygrometric, barometric, and thermometric, — the results 

 of which, being compared with heights accurately determined by spirit-levelling, 

 may furnish data for such purpose. 



But such will require time ; when, however, a considerable number of 

 correct local tables of horary correction shall be produced, by a careful compa- 

 rison of the same, it is not improbable that general tables may be formed, which 

 shall be applicable to extensive portions of the Earth's surface, and by happy 

 combinations of the logarithms of horary corrections with the logarithms of the 

 other coefiicients of the formula, methods of calculation may be contrived, 

 which will give, with little trouble, beyond that of inspection, approximate and 

 closely approximate values of the height of stations of barometric observations 

 one above the other. 



Having now fairly exhausted my subject, I will only add a slight summary 

 of what I have already stated in detail. 



The mode adopted by Laplace of making allowance for the hygrometric 



