128 SECTION PHYSICS. 



in melting, runs into the glass cone which remains at the temperature 

 of o, whilst the excess of water runs through the pipe into an 

 appointed recipient. The moisture of the air in contact with the cold' 

 side condenses itself and covers it with dew, which collecting by 

 degrees into drops, runs towards the point, whence it falls into a 

 graduated vessel. If the time the experiment has lasted be taken 

 into consideration, the quantity of water collected will be found to be- 

 in proportion to the humidity of the air. Experimenting in this wise, 

 the members of the Accademia del Cimento found that the south 

 winds are so charged with moisture that in one minute the hygro- 

 meter has given as much as thirty-five, fifty, and even eighty drops of 

 water, whilst the north wind leaves the glass perfectly dry. 



Here are various examples of the so-called Hydrostamms for 

 liquids, used by the Accademia del Cimento, and some of which we 

 owe to the Grand Duke Ferdinand II. This one with the ballast of 

 mercury is employed for liquids lighter than water ; this other one 

 with the ballast composed of little balls of lead, is for those heavier 

 than water. Just as is usually the case with thermometers, the divi- 

 sions are marked with little balls of glass or enamel soldered to the 

 pipe. Docs not this other hydrostamm, provided with a kind of 

 balancing plate, remind one of Nicholson's aerometer ? It was used 

 for determining the specific gravity of precious stones, by observing 

 to what degree it sank without the precious stone, and with the pre- 

 cious stone placed upon the little metallic disc suspended by the 

 three little chains. This is also a fitting place to draw attention to the 

 so-called " Palla d'Oncia" (ounce-ball) of the Grand Duke Ferdinand, 

 a ball of glass which displaces very nearly an ounce of water ; into its 

 pipe several rings were strung in order to make it sink, and then by 

 the number of these rings it was known what the specific gravity of 

 the liquid was in which it had been immersed. Other hydrostamms 

 were afterwards constructed on the principle of the thermometer 

 with the little balls, which has already been described ; they were 

 called " a gabietta" (like a small cage) ; because the little balls were 

 placed in a sort of cage made of fine brass wirework. When it was 

 immerged into the liquid which was to be experimented on, the 

 number and colour of the balls that sank determined the specific 

 gravity required. And it being known, even at that time, that the- 



