i 97] 



in diameter ; but it would be much better if it did not exceed 

 two and a half; and alfo if it were made, like a plano-convex lens, 

 thin at the edges : indeed the mirror need not be fo broad us the 

 •furface of any fluid ought to be for this purpofe, becaufe to the very 

 edges it is a plane, which is not the cafe with any fluid ; and the 

 contad of the limbs of the images of the fun and moon may, 

 when their altitude is little more than one degree, be diftin- 

 guiflied in a mirror of no greater diameter than two and a half 

 inches. I have experienced that it is eafy to find the reflcded 

 image in a .mirror of lefs than two inches diameter, by advancing 

 the eye with the quadrant as clofe to it as poflible ; and any 

 perfon ufed to make obfervations with a quadrant and to 

 hold it fteady will find this not difiicult. 



The adlion of the wind is exerted more againft the mirror, as 

 being a broad continued furface, and immediately expofed to it, 

 than againft its frame underneath, which is alfo fcreened from 

 it by the fides of the vefl!el ; the top of which, when it is wider 

 than the mirror, fliould perhaps be clofed with fuch a moveable 

 cover, as would fcreen every part of it except that occupied by 

 the mirror, and prevent the wind from blowing down into it. 

 Whatever perturbating force is exerted againft the mirror is at 

 the fame time imparted to the frame, and the momentum of both 

 produces a vibrating motion lafting fome time, and occafions 

 pendulous vibrations of the weight, which are of longer duration 



Vol. VIII. N than 



