466 



DISCUSSION OF BAEOMETKIC OBSERVATIONS. 



dental variations, but we may attempt to eliminate that effect by com- 

 bining them in groups. The following are the mean values of atmos- 

 pheric pressure in ten-thousandths of an inch, obtained directly from 

 Table II, for thirty-six equal periods of 10.15 days each, beginning with 

 January and embracing the entire year. (The integral part, 29 inches, 

 is omitted as before :) 



Table III. 



1. 



2. 



3. 



4. 



5. 



6. 



7. 



8. 



9. 

 10. 

 11. 

 12. 

 13. 

 14. 

 15. 

 16. 

 17. 

 18. 



7018 



6814 



7080 



6992 



7411 



7411 



7996 



7862 



8038 



7456 



7273 



7748 



7394 



7082 



6692 



7171 



7872 



7768 



In this series few oscillations appear beyond the two principal ones 

 already noted. Beginning with the yearly minimum on May 10th, 

 which falls in the thirteenth jjeriod, the rise to the September maximum, 

 in the twenty-seventh, appears to be broken by a depression culminating 

 in the twentieth period (about July 17), and in like manner the fall 

 thence to the November minimum is checked by a subordinate maximum 

 in the end of October, the thirtieth period. The only remaining note- 

 worthy departure from the normal value is more marked than either of 

 the preceding, and consists of a depression in the ninth period — the ten 

 days following the vernal equinox — which appears to amount to a true 

 equinoctial storm. Thus each of the two equinoxes is a definite baro- 

 metric epoch, though they are marked by opposite characteristics. 



A tendency is api)arent in one or both of the periods adjacent to a 

 maximum or minimum to depart from the normal values in an opposite 

 direction, so that the gradual approach to the critical value, on the part 

 of the more distant periods, appears to culminate ten or fifteen days 

 from the true epoch, and a double maximum or minimum is the result. 

 Examples may be noticed in the fourteenth period, immediately follow- 

 ing the yearly minimum in May, while a double maximum occurs in 

 September ; also, in the case of minor oscillations, the minima in March 

 and July are each followed by a single period of high barometer, and 

 the January maximum is both preceded and followed by single periods 

 slightly lower than those adjacent to them. The phenomenon appears 

 suspicious, as though due to accidental variations, yet some grouiid for 



