Mr. Galbraith on Sound, and the Ellipticity of the Earth. 179 



I must apologize for having mentioned a subject which is 

 not astronomical, on account of its intimate connexion with 

 the preceding part of this paper. 



Steinheil. 



XXIX. Comparison of a Formula representing the Velocity of 

 Sound, with Capt. Parry and Lieut. Foster's Experiments 

 on that Subject at Port Bowen ; with some Remarks on the 

 Ellipticity of the Earth. By Wm. Galbraith, Esq. A.M. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 TN the first volume of the New Series of the Phil. Mag., 

 •*- p. 337, I have given two formulae to determine the velocity 

 of sound. It would be more convenient, however, to adapt 

 that for the temperature by Fahrenheit's thermometer to zero 

 of that scale, and it becomes 



V=(102-4225 + 0*11030(l+ gr^:27)(10-2739- 0-0138 cos 2X] 



+ co cos <p. 

 This gives the velocity in English feet, when the English ba- 

 rometer and Fahrenheit's thermometer are used. 



By a comparison of this formula adapted to the centigrade 

 thermometer, I found an almost perfect accordance with Dr. 

 Moll's experiments. I also found that the effect of wind on 

 the 27th of June 1823 was about 19 feet, — half the difference 

 between the velocities, as determined from each extremity of 

 the base. Indeed there can be little doubt that the velocity 

 of sound is affected by that of the wind at the time. Dr. Gre- 

 gory of Woolwich, in a series of experiments on sound de- 

 tailed in the first volume of the Transactions of the Cambridge 

 Philosophical Society*, expressly states that the wind increases 

 or diminishes the velocity of sound according as it blows in 

 the same or in an opposite direction ; — a conclusion which 

 might almost a priori have been anticipated. The only diffi- 

 culty is, to adapt the formula to the actual state of the atmo- 

 sphere with regard to moisture. The expansion of the dry air 

 with which Messrs. Dulong and Petit operated, was 0*375 from 

 the freezing point to the boiling point of water. It is a little 

 greater, however, in moist air, such as exists in an ordinary 

 state of the atmosphere. Laplace in that case assumed 0*4<, and 

 from a mean of a great number of experiments on air, sound, 

 &c. I found 0*4112, that adopted in the above formula. 



* See Phil. Mag. vol. Ixiii. p. 401.— Edit. 



2 A 2 Now 



