296 Royal Society. 



Professor Forbes's empirical formula respecting the gradual diminu- 

 tion of the daily oscillations of the barometer, within certain limit 

 hours, from the equator to the poles. Professor Forbes has laid 

 down an assumed curve, in which the diurnal oscillation amounts 

 to -1190 at the equator and in lat. 64 8' N., and beyond that lati- 

 tude the tide should occur with a contrary sign, plus becoming minus. 

 Now Alten being nearly in lat. 70, if Professor Forbes's law hold 

 good, the maxima of the diurnal oscillations should occur at the 

 hour for the minima at the equator, and a similar inversion should 

 take place with respect to the minima. Mr. Thomas has himself 

 however modified the value his observations would otherwise have 

 had, by adopting 2 P.M., instead of 3 P.M., for the hour of his ob- 

 servations for the fall; and he has adapted his barometrical ob- 

 servations to a mean temperature of 50 Fahr., instead of 32. 

 The first year's observations commence on the 1st October, 1837, 

 and terminate on the 30th September, 1838. The barometer stood 

 66 feet 5 inches above low- water mark, and the thermometer hung 

 at 6 feet above the ground ; but care was not always taken to pre- 

 vent the sun shining on it. The mean height of the barometer 

 for the year was 29'771, and the mean of the thermometer al- 

 most coincident with the freezing point, viz., 32'017. The 

 maximum height of the barometer was 30'S9 in January, and the 

 minimum 28*71 in October. The mean of the barometer at 9 

 A.M. was 29-764, therm. 33'455 ; at 2 P.M. 29'765, therm. 

 33-327 ; and at 9 P.M. 29'784, therm. 29'270. The diurnal 

 observations would seem to support Professor Forbes's theory ; but 

 the 9 P.M. observations are entirely opposed to it, as they appear 

 with the same maximum sign as at the equator, whereas the sign 

 ought to have been the reverse ; indeed, with respect to the diurnal 

 observations, the mean of five months of the year at 9 A.M. gives a 

 plus sign, although the mean of the year at 2 P.M. only gives the 

 trifling quantity of '001 plus. There is one remarkable feature in 

 these observations that cannot fail to strike the meteorologist. M. 

 Arago, from nine years' observations at Paris, reduced to the level 

 of the sea, makes the annual mean height 29'9546 ; twenty-one 

 years' observations at Madras make it 29'958 ; and three years' 

 observations at Calcutta, by Mr. James Prinsep, make it 29'764 ; 

 and Mr. Thomas brings out 29'771. That there should be this 

 coincidence between the observations at Calcutta and Alten is 

 curious. Neither Mr. Thomas nor Mr. Prinsep state whether or 

 not their means are reduced to the level of the sea. It is to be 

 suspected they are not. 



For the next year, that is to say, from Oct. 1838 to Sept. 1839, 

 both inclusive, Mr. Thomas uses a French barometer and French 

 measurements, with centigrade thermometer attached to the baro- 

 meter, and Fahrenheit's for the detached thermometer. He changes 

 his time of observation from 9 A.M. to 8 A.M., 2 P.M., and 8 P.M., 

 and he reduces his barometrical observations to centigrade. 

 The results of the year are as follow : mean annual pressure 

 29-627 English ; thermometer Fahr. 33'36 ; greatest pressure 



