565 



de ce dernier nombre, du moins jusq'a ce que Ton soit par- 

 venu par une longue suite d'observations sur I'hygroinetre, a 

 introduire cet instrument dans la mesure des hauteurs par le 

 bai'ometre."* 



I may in conclusion observe, that in assuming, with the 

 view of calculating the expansion produced by moisture, that 

 the pressure to be employed is the geometric mean of the 

 corrected pressures given by the barometer at the two sta- 

 tions, I am quite aware that I am assigning to it but an ap- 

 proximate value. An exact expression for the pressure to 

 be employed admits of being investigated ;f but its intro- 

 duction into the formula, while it would give the latter com- 

 complexity of form, and thus render it less suited for prac- 

 tical use, would conduct to results not appreciably different 

 from those given by the more simple methods just ex- 

 plained. 



Mr. Clibborn presented to the Academy an ancient stone 

 image, called in some places a Shela-na-gig; and read the 

 following extract from a letter from Dr. Charles Halpin : 



*' About two years ago, as I drove past the old grave- 

 yard of Lavey Church, I discovered this curious figure, laid 

 loosely, in a half rechning position, on the top of a gate pier 

 that had been built recently, to hang a gate upon, at the an- 

 cient entrance of the old church-yard. I believe the stones 

 used in building those piers were taken from the ruins of 



* Systeme du Monde, p. 89. 



t Let — iT\ ' ^ ''^'"S ^"^^ modulus of the common system of logarithms, 



r= p. Then if k be the column of dry air, and that, when saturated with mois- 

 ture whose force is /, it becomes v, we will have 



p 



„'=y X 



Tor the very elegant expression for p I am indebted to my friend, Professor 

 Renny. 



