632 Lieutenant Renny on the Constants of 



HOUR to accomplish a peculiarly important object must be set aside as 

 defective. This brings me to a table of horary corrections calculated by 

 myself, with data extremely correct, for every hour (night as well as day) of 

 every month of the year. My method of calculating this Table is the follow- 

 ing : — With my own correct formula and constant, obtained by the considera- 

 tion of the ratio of specific gravities of dry air and quicksilver, I made no less 

 than two hundred and eighty-eight distinct calculations for every hour, night 

 and day, of every mouth of the year, from the data supplied by M. Planta- 

 mour's quarto, entitled "Eesume" {vide pages 8, 21, 29, 37, 51, and 57). 

 Then subtracting each calculated height from the true height, as ascertained by 

 accurate spirit-levelling, viz., 2070-34 metres {vide tract, "Nivellement du 

 Grand-Saint-Bernard"), the error of each calculated height is known. Such 

 error, being divided by the calculated height, gives the horary correction — for 

 if such horary correction be multiplied by the calculated height, we necessarily 

 have the diflerence of calculated and true height ; and such difference being 

 added to, or subtracted from the calculated height, according to the sign of the 

 horary correction, necessarily gives the true height. Thus, as unity is to unity, 

 plus or minus the horary correction, so is the calculated height (given by for- 

 mula) to the true height. I have added to the horary correction, in Table 

 X., the arithmetic means of temperatures, as given by the detached thermo- 

 meters, which may be found in the quarto, entitled " Eesume," &c., at pages 

 8 and 29 ; and by inspection of this Table many striking peculiarities may 

 be seen connecting these means and the errors of calculated heights, some of 

 which, having important bearings on the subject of the present paper, I pro- 

 ceed to notice. By inspection of the said Table of horary corrections and means 

 of temperatures, it appears that the greatest horary corrections, having the 

 minus sign (— ), which indicate the greatest errors in excess of calculated heights, 

 take place at one hour p. m. ; and that for some months the highest tempera- 

 tures take place at the same hour, but that for other months the greatest heat 

 takes place at 2 p. m. 



It also appears that the greatest errors of calculated heights, in defect, take 

 place after midnight, between 2 and 5 o'clock, and that the connexion between 

 greatest errors in defect and lowest temperatures, occurring during night-time, 

 is by no means so close as the connexion between the greatest errors in excess 



