194 Prof. G. Luvini's Experiments and Observations 



this condition fail, however little, the results are not coincident. 

 I found, however, from a large number of trials, that in the latter 

 case the mean of each pair of experiments gave results perfectly 

 agreeing with those first obtained, even when the index came to 

 rest 2° or 3° from the position of equilibrium. I should notice, 

 however, that in my experiments the torsion of the thread was 

 never less than 10°, but more frequently it was 40 c . Knowing 

 how much torsion to give, abridges in many cases the time re- 

 quired for an experiment. 



I now procured a number of needles and plates of various 

 forms and dimensions of brass, copper, iron, steel, glass, wood, 

 bone, whalebone, &c. The plates were cut, some in the form of 

 rhombs like a magnetic needle, some rectangular, some circular, 

 some in segments of circles, many in the form of a disk with, 

 circular holes, or with radial openings, or with holes in the form 

 of sectors extending to the circumference, still leaving an annular 

 boundary. 



My first trials of these materials were with water, sometimes 

 pure and contained in vessels more or less clear; at other times 

 with certain substances dissolved in it. At first I registered 

 only the most salient results, such as would direct me in giving 

 my experiments a more determined aim. When the results were 

 concordant, I began a regular course of experiments and formed 

 a series of 185, most of the members of which contained eight 

 observations. The only liquids I have hitherto used are pure 

 water, or water with substances dissolved in it, and mercury. 



Although I consider the inquiry scarcely begun, yet the fol- 

 lowing principal results appear to be not unworthy of attention. 



1. That the resistance of the surface of the liquid to the mo- 

 tion of the horizontal plates does not sensibly change, or changes 

 very little, with the amount of immersion of the plate; for 

 whether its lower surface scarcely touch the surface of the liquid, 

 or the half or the whole of its thickness be submerged, the re- 

 sistance it encounters is always the same. It is also the same 

 when the disk is sunk below the level of the liquid, or when the 

 latter is raised by capillarity round the edge of the disk, as also 

 when the disk, having been submerged, is raised above the level 

 and remains in contact therewith by means of the liquid stratum 

 that it carries up. 



2. That on the surface, as in the interior, the resistance of 

 liquids to the motion of solids increases with the time of expo- 

 sure of the liquids to the air, and with the duration of contact of 

 the liquid with the solid. This result does not agree with that 

 of Hagen*, who found the surface-tension of water in contact 



* * Ueber die Oberflache der Fliissigkeiten/ 5 Mejn. of the Academy of 

 Berlin, 1845. 



