

Mechanical Science. 



comprised between 52 32' and 50 7'. The instrument used was on 

 the model of that belonging to M. Hansteen, and employed by 

 Captain Sabine. -The needles were small cylinders of steel, pointed 

 at the extremities, and suspended in glass cases by silk filaments*. 



9. AN ACOUSTIC RAINBOW. 



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 greatest possible intensity of tone, numerous drops 

 of water fly perpendicularly and laterally upwards. The size of the 

 drops is smaller as the tone is higher. The outer and inner rainbows 

 are very beautifully seen in these ascending and descending drops, 

 when the artificial shower is held opposite to the sun. When the 

 eyes are close to the falling drops, each eye sees its appropriate 

 rainbow, and four rainbows are perceived at the same time, particu- 

 larly if the floor of the room is of a dark colour. The square plate 

 on which Professor Strehlke made the experiment was of brass, nine 

 inches in length, and half a line in thickness. The experiment suc- 

 ceeds best tf, when a finger is placed under 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, a fourth of its length from 

 one of its angles. An abundant shower of drops is thus obtainedf. 



10. COMPRESSION OF FLUIDS. (Professor Oersted.) 



From a series of experiments on this subject, M. Oersted was led 

 to the following results : 



i. The compressibility of fluids, up to the pressure of 70 atmos- 

 pheres, is proportional to the pressure. 



ii. Up to the pressure of 48 atmospheres, no perceptible degree of 

 heat was developed in water. 



iii. The compressibility of quicksilver does but very little exceed 

 the millionth part of its volume for every atmosphere. 



iv. 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 water. 



v. Water which contains salts in solution is less compressible 

 than pure water. At 32 F. pure water is by one-tenth more compres- 

 sible than at 55 F. ; at higher temperatures its compressibility also 

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



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

 of quicksilver. 



Mr. Perkins found the compressibility of water more than double 

 that resulting from M. Oersted's experiments ; a difference which, 



* Bull. Univ. 1830, p. 365. 

 f Poggendorff's Annal. 1830, No. 3. 



