230 



maximum height of the barometer was 30°" 89 in Januar}% 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 

 obser^-ations, 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' 

 observ^ations 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, My. 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 

 in April, least in January! ! The mean of 8 a.m. is 29^'620 ; 

 therm. 33°-75. The mean of 2 p.m. is 29''-631 ; therm. 34°-73. 

 And at 8 p.m. 29°'631 ; therm. 30"'57. The diurnal observa- 

 tions assist to support Professor Forbes's theorj'' ; but as in the pre- 

 ceding year, the p.m. observation is at fault; and if the hour had 

 been 9 o'clock instead of 8 o'clock, it would probably have been 

 more so than it appears. The low annual mean state of the baro- 

 meter for the year 1837-38 is even increased in the last year's ob- 

 servations ; and as fresh instruments'^ appear to have been used, 

 there is ground to believe that the fact is associated with the lo- 

 cality, and it may be desirable not only to record in the Proceed- 

 ings of the Royal Society the data already supplied, but to re- 

 commend to Mr. Thomas more particular inquiry on the subject. 



The phsenomena of the Aurora Borealis appear to have been ob- 

 served by Mr. Thomas with great assiduity, and recorded with gi-eat 

 care. On examining the register, with reference to M. Erman's 

 important remark, that " in Siberia two kinds of aurora are distin- 

 guished, one having its centre in the west, and the other in the east, 

 the latter being the more brilliant/' it is found that twent}^-two 



* It appears that the barometer was compared before lea"^ing France, and sub- 

 sequently to its being taken back to that countiy. 



