Intelligence and Miscellaneous Articles. 449 



greater than that which surrounded the moon, and the two arcs of 

 the circles almost joining at their extremities, formed towards the 

 zenith an immense crescent, the effect of which was rendered still 

 more remarkable at the moment by the favourable aspect of the 

 clouds ; within the space comprised between the two semicircles the 

 sky presented only faint striated vapours, while at the exterior parts 

 it was covered for the most part by small woolly clouds. 



Professor Ksemtz, in his ' Meteorology,' remarks that *• as the 

 halos appear most frequently when the barometer falls, they are 

 likely to be followed by rain ;" and in the present instance, on the 

 morning of the 20th of January, the barometer fell more than six 

 millimetres, and a heavy rain fell which continued during the day. 



ON THE MOTION OF ELASTIC FLUIDS, AND THE THEORY OF 

 WIND INSTRUMENTS. BY A. MASSON. 



The author's memoir treats of the flow of elastic fluids through cir- 

 cular orifices pierced in metallic plates ; the acoustic phenomena 

 produced by the flow of air through circular orifices adapted to 

 .cylindrical pipes ; numerous experiments on pipes of different 

 material and length ; an examination of the various theories pro- 

 posed as explanatory of the motion of air in acoustic pipes, together 

 with some remarks on the function of the apertures of organ pipes. 



Flow of air through circular orifices. — A rectangular deal box was 

 suitably placed upon the reservoir of an organ bellows, and on its 

 upper surface were adjusted metal discs pierced at their centres 

 with a circular orifice. A manometer — consisting of a narrow 

 inclined tube communicating at one part with the conduit pipe and 

 at the other with a large flask filled with water — indicated the 

 aerial pressure during the experiment. This differential mano- 

 meter, capable of being inclined in any direction, gives the true 

 pressure by multiplying the variations of the column of water by the 

 sine of the respective inclinations. Several applications of this new 

 apparatus are pointed out in the memoir ; its great sensibility was 

 a main condition of the success of the experiments, which required 

 the application of very minute degrees of pressure, frequently less 

 than that of one millimetre of water. 



The air in flowing through the circular orifices in the metallic 

 plate produces a sound which rises to a higher pitch in a continuous 

 manner with the increase of pressure, as in the syren. 



The sounds which can be obtained from the same orifice are com- 

 prised within two limited pressures, which depend upon its diameter 

 and the thickness of the plate. 



Elastic fluids in passing through narrow apertures acquire a vibra- 

 tory condition ; the number of vibrations which they perform in 

 any case is proportional to the square root of the pressure or to the 

 velocity of their flow, and is not dependent upon the diameter of 

 the orifices. 



Flow of air through orifices surmounted by acoustic pipes. — The 

 periodical motion of the air which flows through the orifices does 

 not always exercise upon the organ of hearing an action sufficiently 

 energetic to give rise to the sensation of sound. 



