O’Toorr—An Improved Form of Hydrometer, Ete. 755 
a second marked point between the third bulb and the dish. 
These additional weights are evidently, according to the well- 
known law of Archimedes, the weight of a volume of the 
liquid equal to the bulb between the two points. In this way 
the weights of the same volume—the volume of the bulb—of any 
two liquids may be determined with extreme accuracy, and their 
relative densities may easily be calculated. The fact of having 
two standard points completely eliminates the effect of surface 
tension, so fatal to the accuracy of all other forms of hydrometer. 
It has a great advantage in that it requires only two spindles of 
convenient size for all liquids from the heaviest to the lightest : 
Twaddell’s form of the common hydrometer requires six spindles 
for heavy liquids alone, and as many others would be required for 
light ones. 
The proposed hydrometer should prove useful in junior ex- 
perimental classes; it 1s inexpensive; a beginner can obtain with 
it the most accurate results, and, above all, the accuracy of the 
result will depend entirely on the worker. 
