468 Pneumatic Analogue of the Wheatstone Bridge. [Apr. 24, 



arranged by altering the quantity of gas burning in the jet. No 

 difference was, however, observed in the position of equilibrium of 

 the needle, whether the gas was quite low, or on full, or turned out, 

 leaving only the head due to the heat of the metal chimney. So far 

 as could be tested in this manner, one of the advantages of the 

 Wheatstone bridge, viz., that the adjustment is independent of the 

 electromotive force, is correctly followed in the pneumatic analogue.* 



III. Comparison of a Circular with a Bectangular Aperture. 



A circular aperture in a brass plate, one-sixteenth of an inch thick, 

 was balanced against a rectangular one, formed by the sliding card- 

 board shutters. The circle was turned to be 1 inch in diameter, and 

 the inner edge of the aperture was bevelled. The observations, when 

 the circle was in the position a lf were — 



a 1 = ttX (i) 2 = -785 sq. in. 



f 489-51 



Readings for a 3 , < 490 > Mean, 490. 



[490 J 



Whence a s = 1*446 sq. in. 



J% _ i-86. = 2. 



a i a 2 



When the circle was in the position a 3 , 



a 3 = *785 sq. in. 



Readings for a } 



Mean, 125-5. 



Whence a-± = "362 sq. in. 



^3 _ 2-17. ^1 — 2. 



a 2 



The observations were repeated with similar results. 



These two values of the ratio a s /a l would be reconciled by assum- 

 ing that the circular aperture was only equivalent to a rectangle 

 whose area is 0'925 of the circular aperture, and they, therefore, 

 throw doubt upon the idea that circular and square apertures have 

 the same coefficient of contraction, but the rectangular apertures were 



* The flow of air through an aperture (a 3 ) of 1 sq. in. amounts to about 2 cubic 

 feet per minute when Jhe gas is very low, and to 4 cubic feet per minute when it is 

 full on, so that the head can be changed in a ratio of about 4 : 1. 



