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XXIII. On a Syphon Hydrometer, and its Use in finding tJie 

 Temperature of Water at the greatest Density. By Mr. 

 Henry Meikle*. 



^I^HIS hydrometer consists of a glass tube, open at both 

 ■*■ ends, and bent into a sort of double syphon, having four 

 parallel legs ; so that the open ends are 

 pointed in the same direction, or upwards, 

 as in the annexed figure. 



The manner of using it is very simple: 

 Let one of the ends be stopped with the 

 finger or with a cork, and water be poured 

 into the other. This fluid will only rise a 

 small way into the second leg, because of the 

 included air. Next stop the other orifice, 

 and open the one first closed; and having 

 poured into the latter the liquid whose spe- 

 cific gravity is to be tried, open the top of 

 the water-tube ; then the instrument being 

 held upright, the two liquids will arrange 

 themselves so as to press equally on the in- 

 cluded ah*. Now this pressure will be mea- 

 sured by the difference in the heights of the 

 two columns of either liquid multiplied by 

 its specific gravity. So that by dividing the 

 difference of the two columns of water by the 

 difference of those of the other liquid, we 

 obtain the specific gravity of the latter ; that of water being 

 unity f. 



The difference between the columns, which is the effective 

 column, may be measured by applying any scale of small equal 

 parts ; or the glass might for greater safety be attached to a 

 graduated board or plate, and this furnished with verniers, &c. 

 Some little attention must be paid to the quantities of the li- 

 quids employed, for the longer the columns are the result will 

 be the more certain ; but no great nicety is required as to the 

 precise quantity of either liquid. It is plain that the expan- 

 sion of the glass or its capillary action have nothing to do with 

 the result. Nor can the expansion of the scale have any in- 

 fluence ; because the ratio of the columns is not altered thereby. 

 Only if the temperature differ from the standard, as from 40° 



* Communicated by the Author. 



-J- The difference of the weights of the columns of air is neglected, as of 

 no consequence in practice. 



Fahr. 



