CONTRACTION OF WATER BY HEAT, 



36% 



Nicholfon fays, It is fenfible to the 20th part of a grain*. 

 The one I have, though its ftem be flender, is fcarcely fenfible 

 to lefs than two or three twentieths of a grain. 



The want of fenfibility in the gravimeter arifes, in a great 

 meafure, though not entirely, from a certain degree of tenacity 

 fubfifting among the particles of the fluid ; and any thing that 

 tends to increafe this tenacity, muft, in the fame proportion, 

 augment this want of fenfibility. 



To afcertain whether any fenfible change in the tenacity or it rofe and fell 

 fluidity accompanies the expanfion of water by cold, which the ,nwarm > an * 

 theory requires, I examined the mobility of the'inftrument 

 when immerfed in water at different temperatures. I flrft 

 plunged it into this fluid, heated to between 60° and 70*. 

 Under due loading, which funk it to the mark on the ftem, 

 it was not fenfible to a weight lefs than two or three twen- 

 tieths of a grain, - 



I then tried it in ice-cold water, and found that its fenfibility in jce-coldwatet 

 was in no perceptible degree impaired. The coldnefs of ihe^j lit ^ ua 

 water, it muft be remembered, caufes fome degree of contrac- 

 tion of the gravimeter. This contraction cannot fail to render 

 the inftrument in fome fmali meafure more fenfible, and, fo far 

 as it goes, to counteract the fluggiflinefs produced by any in- 

 creased tenacity in the fluid. 



But as the body of the inftrument is made of glafs, the Whence the 

 amount of the contradion muft be very fmall, and the change £jjj^5£ 

 of fenfibility arifing from it fo very trifling, as certainly by no is not fenfibly r 

 means to obfeure fuch an effect as an increafe of tenacity would chan 2 ed » 

 occafion. I therefore with fome confidence conclude, that the 

 fluidity of the water is not fenfibly diminifhed, and con- 

 fequently that the polarity has not begun to exert any fenfible 

 influence ; it can fcarcely, therefore, be accounted the caufe 

 of the dilatation. 



* Perhaps the difference of fenfibility in my inftrument, and that 

 of the learned Profeffur, may have arifen from a difference of 

 the diameters of the ftems. Mine was of one-fortieth of an inch. 

 It was well rubbed with a clean linen cloth, which rendered the 

 furface equally difpofed either to defcend or afcend ; and the in- 

 ftrument was not judged to be in equilibrio with the fluid, except 

 when the furface about the ftem was neither prominent nor de- 

 preffed. This was eafily known hy the reflected image of the 

 window frame, or other objects being ken clofe to the ftem with- 

 out diftortion.— N. 



h ANNOTATION, 



