Miscellanies. 381 



but will be pressed upwards against the hand as long as the current 

 continues. The moment it ceases, the paper falls by its gravity. 

 M. F.—Joiir. of Roy. Inst. 1. 368. 



6. An acoustic rainbow. — (Poggendorf's Annals.) Professor 



Strehlke states that a sounding plate, covered with a layer of water, 

 may be employed to produce a rainbow in a chamber which admits 

 the sun. On drawing the violin bow strongly, so as to produce the 

 strongest possible intensity of tone, numerous drops of water fly per- 

 pendicularly, and laterally upwards. The size of the drops is smaller 

 as the tone is higher. The outer and inner rainbows are very beau- 

 tifully seen in these ascending and descending drops. Each eye 

 sees its appropriate rainbow, and four rainbows are perceived at the 

 same time, particularly if the floor is of a dark color. The square 

 plate was made of brass nine inches in length, and half a line in thick- 

 ness. The experiment succeeds best if, when a finger is placed un- 

 der the middle of the plate, and both the angular points at one side 

 are supported, the tone is produced at a point of the opposite side, 

 one fourth of its length from one of its angles. An abundant shower 

 of drops is thus obtained. — Idem. 



7. Compression of fluids. — From a series of experiments on this 

 subject, Prof. Oersted was led to the following results : — • 



1. The compressibilitj^ of fluids up to the pressure of seventy at- 

 mospheres, is proportional to the pressure. 



2. Up to the pressure of forty eight atmospheres, no perceptible 

 degree of heat was developed in water. 



3. The compressibility of quicksilver does but very little exceed 

 the millionth part of its volume for every atmosphere. 



4. The compressibility of sulphuric ether is three times as great 

 as that of alcohol, twice that of sulphuret of carbon, and one and a 

 half that of Vv ater. 



5. Water which contains salts in solution is less compressible than 

 pure water. At 32° F. pure water is by one tenth more compressi- 

 ble than at 55° F. ; at higher temperatures its compressibility also 

 decreases, though not "to such an extent as between 32° and 55°. 



6. The compressibility of glass is very small, much less than that 

 of quicksilver. 



Mr. Perkins found the compressibility of water more than double 

 that resuhing from Mr. Oersted's experiments ; a difference which, 



