^70 



C'ATAEOGUE. — ACUSTICS, EFFECTS' OP SOtTNlX 



no variation during tvU this time-; wo were* absolutely cer- 

 tain, that whatever motion wa» perceived afterwards, was 

 wholly owing to the sound. 



Mr. Pigott, who was then at Brussels, was kind enough 

 to lend me one of his barometers, made by Ramsden, and 

 his son made the following observations jointly with myself. 

 At two o'clock in the afternoon of the first of November, 

 1773, we went into the northwest tower of St. Gudula's 

 church, and having fixed the barometer firmly in the open- 

 ing of a window, not above seven feet from the bottom of 

 the bell, we waited quiedy for its ringing. 



The height of the mercury before the bell began to swing, 

 as observed by Mr. Pigott, was 29.478 inches. The bell 

 being iri^fuU swing no alteration whatever was perceptible. 



The instant that the clapper was loosed, the mercury 

 leaped up, and continued that sort of springing motion at 

 every stroke of the clapper, during the whole time of the 

 ringing of the bell. These were our observations. 



During the ringing of the bell, Mr. P. 29.409 



During the ringing, by myself 



Highest 29.480 



Lowest 29.474 



Highest 29.482 



Lowest 29.472 



These observations were made with the greatest atten- 

 tion ; and considering their delicacy and the difBculty of 

 observing, agree very nearly. They appear to give from 6 

 to 10 thousandths of an inch for the effect of this sound on 

 the barometer. It is to be observed, that Mr. Pigott in 

 general, estimated the height of the mercury about five 

 thousandths lower than myself, which brings our observa- 

 tions to a very near agreement. The following observations 

 prove this. 



On the top of the tower, Mr. P. 29.424 



Ditto, by me 29.430 



At the foot of the tower, Mr. P. 29.639 



Ditto, by me 29.642 



In the court of the English Nuns, by Mr. Pigott 29.676 

 Ditto, by me 29.682 



And I should think, that the difference of eyes may fre- 

 quently cause Euch a variation among different observers; 

 at least, in delicate observations, it will be always prudent 

 to make the experiment. 



Note ly Dr. Young. 

 These observations appear to agree too well with each 

 other, to allow us to doubt of their accuracy. It therefore 

 becomes necessary to inquire after the cause of the differ- 

 ent heights of the barometer. It is indeed barely possible, 

 that a sudden stroke of the clapper of the bell might pro- 



duce a greater agitation of the buiMiof^ than the preceding 

 alternate motion of the bell itself: but this explanation 

 cannot be called satisfactory. It is certain, that there was 

 neither more nor less air in the tower while the bell waj 

 sounding, than while it was silent ; the mean density of the 

 air could therefore not have been changed ; and if the al- 

 ternate motions of the particles of air which constitute 

 sound, had ttken place by equal degrees and with equal 

 velocities in each opposite direction, there is no reason to 

 suppose that the increase of pressure on the surface of the 

 mercury, at one instant, could have tended to raise it, more 

 than the decrease of pressure, in the opposite state of the 

 undulation, would have depressed it. But the same conse- 

 quence does not follow, if we conceive the motion of the 

 air in advancing to be more rapid, but of shorter continu- 

 ance, than its retrograde motion. For if the wind blew for 

 one hour with a velocity of four, and the same air returned 

 in the course of two hours with a velocity of two, an ob- 

 stacle upon which it had acted in both directions, would not 

 be found in its original place ; for the action of the wind 

 upon anobstacle, is as the square of the velocity, and the 

 time would not compensate for the difference of force. It 

 is therefore easy to suppose, that the law of the bell's vi- 

 bration was in this experiment such, that the air advanced 

 towards the barometer wiih a greater velocity than it re- 

 ceded, although for a shorter time, and that hence the 

 whole effect was the same as if the mean pressure of the air 

 had been increased. Such a law might easily result from a 

 combination of a more regular principal vibration with one 

 or more subordinate ones, in different relations ; and simi- 

 lar cases may sometimes be observed in the vibration* of 



chords. 



* 



Sympathetic Sounds. 

 Morhofii stentor hyaloclastes, de scypho vitreo 



fracto. Kiel, 1662. 1683. 

 Wallis on the paitial sympathetic tremors of 



chords. Ph. tr. 1677. XII. 839. 



Shown by bits of paper laid on the chords. 



Lahire ou a buttress at Rheims that vi- 

 brates when one of the bells is rung. A. 

 P. II. 87. 



Probably from being capable of vibrations equally fre- 

 quent with the swinging of the bell. 



*Ellicotton the mutual influenceof twoclocks. 

 Ph. tr. 1739- 126. 



A curious instance of sympathetic vibration ; the motion 

 of one of the clocks put the pendulum of the other in mo- 



