402 Dr. Gregory on the 



The theoretical formula most generally adopted, especially 



by continental philosophers, is this : 



Velocity in horizontal direction = 333-44. met. */ 1 + "00375^; 

 the metre being =3*2809 English feet, and t denoting the 

 indication of the temperature upon the centigrade thermo- 

 meter. 



I am inclined, however, to think that this can only be re- 

 garded as an approximative formula ; and that we are not yet 

 in a state to receive otherwise than as an approximation any 

 theorem which simply includes the variations of temperature. 

 The air is subject to various classes of changes, indicated by 

 the barometer, thermometer, hygrometer, and anemometer 

 respectivel}*, as well as others probably, for the ascertaining 

 of which we have not yet any appropriate instrument. If we 

 could select these, one by one, ad libitum, and carry experi- 

 ments first through a moderate range upon the barometric 

 scale, all the other probable elements of modification remain- 

 ing constant; then, through a sufficiently extensive range 

 upon the thermometric scale, the others remaining invariable, 

 and so on ; the question would soon be set at rest : but this is 

 impossible. It becomes desirable, therefore, to augment the 

 number of recorded facts, as they result from accurate experi- 

 ments, in order that at some future (and it is hoped no very 

 remote) time a cautious investigator may so select, compare, 

 and classify them, as to deduce a more comprehensive and 

 accurate theorem than is yet known. 



With a view to contribute, though in a small degree, to this 

 purpose, I now present an account of a few experiments made 

 by myself in the course of the present year. 



My objects were, to ascertain the velocity with which the 

 sound passed over the surface of the earth, over the surface of 

 the water; under different temperatures; in a quiescent state 

 of the atmosphere, and in windy weather; by day and by 

 night ; the velocities of direct and reflected sound ; and the 

 velocities of sounds of different intensities and produced by 

 different means. As yet the experiments have not been car- 

 ried to their projected extent ; but while I record the results 

 thus far obtained, I lcok forward with hope, that in another 

 year or two I shall be able to complete them satisfactorily. 



The instrument with which I measured the intervals of 

 time, was one invented and made by Mr. Hardy, by means of 

 which, with a little previous practice, I could measure an in- 

 terval accurately to a tenth of a second, and approximately 

 to a twentieth of a second. The velocity of the wind was as- 

 certained by means of an anemometer ; and the barometer 



and thermometer were of the best construction. T 



I em- 



