IN OPTICS. 73 



upon firm ground, at right angles to the horizon. 

 If, while we are standing erect, it be inclined 

 upon its lower extremity, successively backward 

 and forward, to the right and to the left, these 

 motions must, without contradiction, be per- 

 ceived. Suppose now, our bodies to be simi- 

 larly inclined with the pole, during its different 

 positions, so as to be constantly parallel to it ; 

 it is evident, that its motions will be as readily 

 perceived in this case, as they were, when our 

 bodies were erect ; and this is all that happens 

 in the experiment of Mr. Melvill. 



Should the necessity of supporting the body 

 against its gravity, by the actions of our volun- 

 tary muscles, be suspended in whole, or in part, 

 our judgments of the situation of objects, with 

 respect to the horizon, must become irregular 

 and uncertain, notwithstanding any general 

 habit we may have acquired from experience. 

 An instance of this, I think, I have observed; 

 for I have frequently remarked during a sea 

 voyage, that, when the wind blew so strongly, 

 and in such a direction, as to occasion the vessel 

 to heel, or lean much to one side, chords freely 

 suspended from the roof of the cabin, and kept 

 Stretched by heavy bodies attached to them, 

 have appeared to me, as long as I lay in bed, 

 though they were necessarily perpendicular to 

 the horizon, to decline considerably from that 



