114< Tlie Rev. Professor O'Brien on the Besistance of 



maxima and minima of the diurnal oscillation of the mercury in the 

 barometer. 



Secondly, in respect to the annual variation : 



The annual march is somewhat less regular at Greenwich than at 

 Toronto, being derived from the observations of a single year only ; 

 but we have the same general features : a minimum of temperature 

 and vapour- pressure, and a maximum of gaseous pressure in the mid- 

 winter ; and a maximum of temperature and vapour pressure, and a 

 minimum of gaseous pressure in the midsummer. All the summer 

 months are characterised by the + sign in the vapour, and by the 

 — in the gaseous pressure ; and all the winter months by the — sign 

 in the vapour, and the + sign in the gaseous pressure. 



I am unable at the present moment to pursue the comparison of 

 the Greenwich and Toronto results in many other points in which I 

 can perceive that the interest would prove an ample repayment for 

 the time so employed. But I may hope to enjoy some future occa- 

 sion of resuming the subject under more favourable circumstances 

 in respect to leisure than I can at present command. 



XI. O71 ike Law of Besistance of a Medium to small vibratory 

 Motioti ; the Mixture of Prismatic Colours ; and the Ap- 

 pearance of the Prismatic Spectrum when viewed through a 

 plate of common blue glass of proper thickness. By the Rev. 

 Prof. O'Brien, late Fellow of Caius College, Cambridge. 

 [The subject resumed from vol. xxv. p. 528.] 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 25. T3 EFORE I proceed to develope more particularly the 

 ■^-^ theory which I have ventured to propose, in a ge- 

 neral way, in your Journal for November and December (Sup- 

 plement) last, I wish to make a few additional remarks on the 

 subject of Resistances, in order to show that I have assumed 

 nothing in the preceding articles but what is warranted by the 

 results of accurate experiments ; and this is the more neces- 

 sary, because the mathematical theory of motion in a resist- 

 ing medium is too imperfect to be of any use, and therefore 

 we must look to experiment alone, in the present state of 

 science, for whatever information we require in this difficult 

 subject. Fortunately the accurate experiments of Borda and 

 Bessel, on the small oscillations of the pendulum in a resisting 

 medium, are abundantly sufficient for our purpose. 



26. By these experiments the following facts have been 

 clearly established : — 



1st. That when a pendulum, consisting of a ball suspended 

 by a fine string, oscillates in a resisting medium through a 

 small angle (for instance, an angle not exceeding 40'), the 



