218 Mr. Galbraith on the Velocity of Sound. 



struments, to determine the exact quantities which ought to be 

 introduced. These are, a good barometer, thermometer, hy- 

 grometer, and anemometer. DanielFs hygrometer gives by 

 means of Dalton's table the elastic force f of aqueous vapour 

 most readily. By the anemometer the velocity of the wind 

 must be determined, and the angle between the direction of 

 the sound and that of the wind must be carefully noted. 



I would not have endeavoured to recommend a complete 

 apparatus so carefully, had it not frequently happened that in 

 pretty extensive series of operations, a necessary part of it, 

 comprehending the hygrometer and anemometer, has been 

 neglected. Of course a set of experiments made in such a 

 manner cannot be strictly compared with our formulae. 



In an analysis of Mr. Goldingham's experiments made at 

 Madras, I endeavoured, in a former Number of this Journal, 

 to point out the necessity of attending to all the circumstances 

 likely to affect the accuracy of such experiments. Since that 

 time, another series has been made lately by Captain Parry 

 and Lieut. Foster, in which the same omissions occur ; and of 

 course no accurate conclusion can be drawn with respect to 

 the effects of barometric pressure and temperature. This is 

 the more to be regretted, as in the hands of such accurate 

 observers, in such a climate, where the temperature was fre- 

 quently very low, formulae answering very well for the usual 

 state of the atmosphere would have been put to a severe test. 



If all the circumstances to which allusion has now been 

 made had been attended to, Messrs. Parry and Foster would 

 undoubtedly have detected the mistake into which they have 

 apparently fallen. In the Appendix to Captain Parry's Journal 

 just published, page 86, there is given a table of the velocity 

 of sound from observation at Port Bowen. In determining 

 these, the states of the barometer and thermometer alone are 

 marked. At such low temperatures it frequently happens that 

 no moisture can be detected by Daniell's hygrometer, according 

 to the testimony of Lieut. Foster, and consequently its effects 

 on the velocity of sound in such a climate cannot be estimated. 

 It is to be regretted, however, that the velocity of the wind 

 and its direction relative to that of the sound were not ascer- 

 tained ; for if these had been attended to, theory and obser- 

 vation would probably have sonearly agreed, as tohaveprevented 

 these ingenious gentlemen from publishing opinions totally at 

 variance (if I understand them) with those of men of the first 

 eminence. The passage I have in view, is this : 



" These observations appear to indicate a decided decrease 

 of velocity with an increased density of the atmosphere, all 

 other circumstances being alike." Now this opinion is in di- 

 rect 



