4 DISPLACEMENT INTERFEROMETRY. 



or the heating produced was 2Xio~ 4 C. per second. Whether, supposing 

 A A' to be filled with water, a pyrheliometer may be constructed on this prin- 

 ciple I have yet to learn. 



Other interesting results of the same kind might be mentioned. Thus, if 

 the screw stop-cock, figure 2, is closed quickly, there is always a decided 

 increment of pressure. In other words, in consequence of the viscosity of 

 air, the fine space at the plug is virtually a closure before the screw is 

 checked by an actual cloture. 



5. Acoustic pressure, etc. If the glass plates G are removed and the 

 air-space A partially closed with a pipe P (fig. 3) tapering to a neck, PAM 

 becomes a closed organ-pipe with a bottom sensitive to pressure. The pipe 

 may be blown with the adjustable embouchure / described in 45, and suf- 

 ficient free space remains for the component rays U L" of the interferometer 

 to reach the mercury MM'. The neck should not much exceed an inch in 

 diameter. 



It was hoped that when the pipe was sounded judiciously, acousticpressure, 

 if any, would be shown at the bottom. A large number of experiments were 

 made, at first with very weak pipe-notes corresponding to a very mild jet /. 

 Displacements of 5 to 10 fringes nevertheless occurred in the direction of 

 pressure, i. e., up to about 4 dynes/cm. 2 . In later experiments, however, suc- 

 tions were quite as often obtained as pressures, so that what is registered here 

 is the pressure effect of the nearly horizontal air-current from / across the top 

 of the pipe. With strong notes the fringe displacement was much greater and 

 the slit-images tended to separate, destroying the fringes. The results ob- 

 tained, therefore, are merely those of a pipe sounding under slightly reduced or 

 slightly increased pressure, and all attempts to eliminate the discrepancy 

 failed.* Resonance was ineffective. 



The same effect is nicely shown with a thistle-tube t (fig. 4) , provided with 

 a small-bore (o. i cm.) stem c, slightly raised as shown. If a jet from a pin-hole 

 is blown in axially, as at a, the effect is pressure ; if blown in obliquely or tan- 

 gentially, as at 6, the effect observed is suction. The displacements were 10 

 or more fringes, to the right or to the left. 



* The method which ultimately succeeded is described in Chapter III. 



