446 Lieutenant Eenny on a new Barometric 



vations should always, if possible, be made in connexion with barometric ones, 

 but when this cannot be done, it is better, I believe, to obtain from an approved 

 table of forces of aqueous vapour approximate values of/ and/', such as belong 

 to temperatures 3° or 4° below the temperatures t and t\ than to neglect alto- 

 gether the hygrometric state of the atmosphere. With respect to the quantity 

 K{\ + ^ cos 2i^), we may substitute for it a constant calculated for a particular 

 latitude, which will answer well enough for latitudes no more than one degree 

 north or south. Such constant may be designated a Local Constant, and the 

 formula containing it a Local Formula. 

 Let us now reproduce our formula — 



t + t' -U] 



5'=^.il + ^cos2f|.jl + ?^j.jl + ;. 



/3(l + ?^)-§y(//) 



Let us replace K(\+^ cos 2-f ) by a constant calculated for a particular lati- 

 tude. Let such constant be iV, then 



•^^'"•^"g /3'ii-;m(7--V)i-tV(y/) - (B) 



Let us now omit the quantities ( 1 + 1 and I 1 + 1 ; when such 



may be done without serious error, we have 



K is the constant belonging to latitude 45° f = 0002695 ; r = 20898240 feet ; 

 I = 0-002084 ; m = O'OOOl. The term ■/(//"), which is the geometric mean of 

 forces of aqueous vapour, is readily obtained from an approved table offerees of 

 aqueous vapour. 



