337 



that few observers at the distance of fourteen miles could detect 

 it with the naked eye. while the light from the zones was large 



and distinct. 



3. That the rapid passage of the visual object over the eye 

 causes this decrease in its volume, by diminishing the amount 

 of irradiation, which, according to the theory of M. Plateau, is, 

 within certain limits, proportionate to the duration of the impulse 

 of light in the retina. 



The following Donations were presented : — 

 Mfemoire de la Soci6te Geologique de France. Tome iv. P- 1. 



— By the Society. 

 The Transactions of the Royal Irish Academy. Vol. xix. Part 1. 



— By the Academy. 

 Comntes Rendus Hebdomadaires des Stances de TAcadenue des 



Sciences. 1841. Nos. 6, 7, 8. 9. 10.-% the Academy. 

 Proceedings of the Geological Society of London, ^os. 74 and 



75. — By the Society. 

 The American Journal of Science and Arts. Conducted by Pro- 

 cessor Silliman ; for January 1841.—% the Editor. 

 E'tudes Geologiques dans les Alpes. Par M. L. A. Necker. 



Tome i.—By the Author. 

 Maps of the Ordnance Survey of England and Wales. JNos. 75, 



76, 79, and 82— By tlie Board of Ordnance. 



19/A April 1841. 



The Right Hon. Lord GREENOCK, V. P. in the Chair. 



The following Communications were read :— 



1 On the Theory and Construction of a Seismometer— an 

 instrument for Measuring Earthquake Shocks and other 

 Concussions. By Professor Forbes. 

 The plan of this instrument was submitted amongst others to 

 a Committee of the British Association appointed to devise means 

 for registering earthquake shocks. A heavy pendulum, suspended 

 from a frame, will evidently have its bob left behind by its 

 inertia when the frame is moved forwards by any concussion. 

 To render such an instrument very sensible, however, the pendu- 

 lum must be of great length, which presents many mconve- 



