858 Mr. Birt on the great Symmetrical Barometric Wave. 



Helstone being the station of greatest symmetry. The south- 

 ern counties of England will under these circumstances present 

 an area for the establishment of highly important stations. 

 At the Orkneys (the extreme northern station in the series of 

 observations last year), the symmetry was most strikingly de- 

 parted from by the anterior portion of the wave being con- 

 siderably higher than the posterior; while at Paris, the most 

 southern station from which observations have yet been received, 

 the opposite of this occurred, the anterior portion being the 

 lowest and the posterior the highest. These phaenomena are 

 strictly in accordance with the notion of a wave coming up 

 from the north-west and passing on towards the south-east, 

 the direction in which the great symmetrical wave of 1842 

 travelled : as the anterior slope or front of the wave approached, 

 the barometric readings would be lowest at the south-eastern 

 stations, while the reverse would occur as the posterior slope 

 or back of the wave passed off; the readings in the north-west 

 would be lower than those in the south-east. 



On its last return observations were made on the great 

 wave at thirty-four stations extending from the Orkneys to 

 Paris, and from the west of Ireland to Heligoland : should it 

 return during the present autumn, by multiplying observations 

 on it, especially in the southern part of England, we shall be 

 in a much better position to examine it under every possible 

 aspect. The Honourable the Corporation of the Trinity 

 House have directed observations to be made at several light- 

 houses, and Capt. Beaufort has kindly undertaken to obtain 

 observations from several of our surveying vessels. In the 

 Philosophical Magazine for September 1845, the reader will 

 find instructions for making the necessary observations ; and I 

 shall be most happy to forward copies of these instructions, with 

 the requisite forms for recording the observations, on being 

 applied to for that purpose. Observations from any locality 

 will be valuable, but those from any part of England or Wales 

 included by two lines, the northern extending from Dublin to 

 Harwich, and the southern from the Scilly Isles to Calais, will 

 be of great importance in order to determine the direction of 

 the line of greatest symmetry, and to ascertain the character 

 and amount of its variations as compared with its position in 

 former years. 



In addition to the great symmetrical wave, I beg to solicit 

 the attention of your readers to a most remarkable depression 

 of the barometer on or about the 28th of November, which has 

 been observed at Dublin in fifteen out of seventeen years. 

 For the knowledge of this depression I am indebted to Capt. 

 Larcom of the Royal Engineers, who has most kindly furnished 



