1871.] On a supposed Alteration of Aberration of Light §c. 35 



son's cups. The personal equation, under the circumstances of the observa- 

 tions, was found as follows : — A gun was fired at such a distance from 

 the Observatory as to be heard with about the same degree of distinctness 

 as the time-gun at the Castle. This distance was found to be 1483 feet. 

 The registrations on the chronograph were made by Mr. Kirby at the 

 distance of 162 feet from the gun, and Mr. Mann at the Observatory. For 

 this comparatively small distance, the time of transit calculated from the 

 velocity deduced from the time taken to travel over the larger distance 

 may be deemed exact. The observed time for the smaller difference of 

 distance was found to be too great by S *09, which correction has been ap- 

 plied to all the observations. It depends more on want of sensibility in 

 picking up and recognizing faint sounds than upon mere habit of making 

 contacts. "When the observers were interchanged, the observed interval of 

 time appeared still too large, but in this case by s " 02. It is clear that 

 such personal equations are not eliminated by an interchange of ob- 

 servers, nor by return signals. 



In the reduction of the equations, the coefficient of elasticity of air 

 under a constant volume (that is to say, the ratio of the increment of 

 pressure for an increment 1° F. of temperature to the pressure at 32° F.) 

 was regarded as an unknown quantity as well as V, the velocity of sound 

 at 32° F. The reduction of the equations furnished by the observations, 

 which were 38 in number, gave 



V=1090'6 feet per second, 

 a = 0-0019, 



Regnault's value of a being 0*0020. 



There appeared to be but little difference between the residual errors 

 as dependent on the motion of the air. The author grouped the residuals 

 into two classes, according to the dampness of the air ; but there appeared 

 to be no appreciable difference in the velocity as dependent upon damp- 

 ness. 



IV. " On a supposed alteration in the amount of Astronomical 

 Aberration of Light, produced by the passage of the Light 

 through a considerable thickness of Refracting Medium." By 

 George Biddell Airy, C.B., Astronomer Royal. Received 

 November 17, 1871. 



A discussion has taken place on the Continent, conducted partly in the 

 ' Astronomische Nachrichten,' partly in independent pamphlets, on the 

 change of direction which a ray of light will receive (as inferred from the 

 Undulatory Theory of Light) when it traverses a refracting medium which 

 has a motion of translation. The subject to which attention is particularly 

 called is the effect that will be produced on the apparent amount of that 



D 2 



