BAEOMETRIC WAVES. 117 



stages, but this will suffice to show that this important topic was inves- 

 tigated when the means of doing so were much more difficult to obtain, 

 and that some of the decisions now accepted were clearly pointed out in 

 former times. 



In the course of Mr. Birt's researches on atmospheric waves, he had an 

 opportunity of testing the correctness of Professor Dov6's suggestion, which 

 he found to be in close accordance with the truth, for he not only ascertained 

 the existence of the S.W. and N.E. compensating currents, but also that of 

 another set of oppositely directed and compensating winds at right anglea 

 to them. These were N.W. and S.E., with a lateral motion towards the 

 N.E. He also carefuUy discussed the barometric phenomena with relation 

 to both these sets of currents, and arrived at conclusions which the reader 

 will find in detail in Mr. Birt's third report, presented to the British 

 Association for the Advancement of Science (Eeport, 1846, pp. 132 to 162), 

 but which may be briefly enunciated as follows : — During the period which 

 passed under the examination of the writer, he found the barometer gene- 

 rally rising with N.E. and N.W. winds, and that as a maximum or highest 

 reading of the mercury approached, the wind died away mostly to a calm. 

 On the other hand, with S.W. and S.E. winds, the barometer generally 

 fell, the force of the wind proportionally increasing until the mercury 

 passed its minimum or lowest reading. 



There is one important feature which the study of the Barometer has 

 brought to light, and which is by no means devoid of significance, viz., 

 that its oscillations are much greater in the neighbourhood of water. Thus 

 it appears that the junction lines of land and water form by far the most 

 important portions of the globe in which to study atmospheric waves; for, 

 in the great systems of European undulations, it is well known that these 

 oscillations increase, especially towards the N.W., and the late Professor 

 Daniell shewed from the Mannheim observations that small undulations, 

 having their origin on the Northern borders of the Mediterranean, have 

 propagated themselves Northward ; and thus, but in a smaller degree, the 

 waters of the Mediterranean have contributed to increase the osciUationa 

 of the barometer, like the larger surface of the Northern Atlantic. 



From such considerations as these, it becomes very desirable that our 

 knowledge of the rise and fall of the Barometer, in immediate connexion 

 with the direction and force of the wind on the surfaces not only of our 

 large oceans, but also of those of our inland seas, and especially in the 

 neighbourhood of extensive archipelagoes, should be increased. 



(38.) In a future page, in the Section devoted to a description of tha 

 Winds encountered in the Northern part of the Atlantic, this subject o< 

 Atmospheric Waves will be again adverted to ; and then it will be mora 

 plainly seen how it is applicable to the daily experience of the navigator. 



(39.) Baeometeic Gbadients. — The primary principle, that the atmo. 

 sphere moves in a series of waves, indicated by the high or low pressure 

 of the barometer, having been established, the next step was to estimate 

 the dimensions of these waves, and the relation of this element to the 

 changes in the wind's direction and force. In this enquiry several observers 

 recorded the results of their work, including Mr. W. R. Birt, before men- 

 tioned. Dr. Lloyd, in his " Notes on the Meteorology of Ireland, 1854," 



