Don Bustamente on a new Gravimeter, 211 



water (§ 2), and from this construction results the convenience 

 of being able to place the substance on this plate, without 

 taking the whole instrument out of the water, and without 

 being exposed in the second immersion to any bubbles of air 

 rising, which was not the case in the first, but which often hap- 

 pens with Nicolson''s balance, which alters the results. 



12. The level of the water always leaves some doubt in the 

 determination of the precise point of the scale to which it 

 reaches, chiefly when it is to mark the parts of a division ; and 

 in its stead we could use another index much more exact, the 

 simplicity of which recommends its constant use. It consists 

 in placing two threads of silk, ah, c d, (Fig. 3.) well stretch- 

 ed, or of very fine wire, called hair-wire, on the opposite points 

 of the edge r s, of the box or case, so as to encircle the glass 

 tube without pressing it, and allowing it to move freely in the 

 middle, for which purpose are placed the buttons m, n, soldered 

 strongly to the case, and the small grooves a, c, b, d, made on the 

 same edge ; then observing through the hollow x z, which is 

 about two lines high and one inch long, the level of the threads 

 and also of the scale, the thread that is on the side of the ob- 

 server marks the divisions and parts of each to which the im- 

 mersion reaches, and by these means, if the scale is of milli- 

 metres, (French measure,) may be seen at one view the fifth 

 part of each, or two-tenths of a millimetre, which is equivalent 

 in the instrument which I use to a weight of 0.3 grains. It 

 is true that this mode indicates the point of partition a little 

 above zero in the scale ; but as we have seen, (§ 10,) this does 

 not alter the results. 



13. Till now we have only spoken of the mode of weighing 

 substances, the specific gravity of which is greater than that of 

 water. We have then two other cases to consider that can oc- 

 cur, and they are those in which the specific gravity of the sub- 

 stance is equal to or less than that of water. 



14. If knowing the weight of a body in air, for example 24, 

 we weigh it in water, and find that the immersion reaches ex- 

 actly to zero, in this case we should say that it had lost all 

 its weight, or that it is equal to the body of water which it dis- 

 lodges, because the difference between zero and 24 is 24, and 

 its specific gravity will be || = 1 = to that of the water. 



