512 



fected by geographical position and by local circumstances ; and the 

 other phenomena of climate. 



" 2. The effect of season (combined with the influences already 

 referred to) upon the distribution of temperature, and the varying 

 position of the isothermal lines from month to month. 



" 3. The non-periodic variations of pressure, temperature, and 

 humidity, and their connexion with the course and direction of the 

 aerial currents. 



" 4. The phenomena and laws of storms, whether revolving or 

 otherwise. 



"5. The periodical winds prevailing during certain seasons, and 

 their modifications from geographical position or local causes. 



" 6. The course and rate of progress of atmospheric waves. 



" The chief conditions for the solution of these questions are, 

 that the observations should be taken at equal intervals of time, 

 at a sufficient number of well- selected stations, and that the times 

 should be chosen in such a manner as to furnish the daily means of 

 the elements sought. But it has been shown that any two obser- 

 vations in the day, taken at equal intervals, are sufficient to elimi- 

 nate the diurnal variation, and to give the daily means of all the 

 meteorological elements, excepting the atmospheric pressure ; and, as 

 the diurnal variation of the pressure is very small in these latitudes, 

 — much smaller than the irregular fluctuations of the same element, 

 the objects for which the present system is instituted will be best 

 attained, by taking two observations in the day, at homonymous 

 hours. 



" The best pair of homonymous hours, all circumstances consi- 

 dered, is 9 A. M. and 9 p. M., mean time of the place of observation ; 

 and these are accordingly proposed as the fixed hours of observa- 

 tion for those who co-operate in the present system. In order to 

 elucidate more fully some of the questions above noticed, and in 

 particular that which relates to the movements of atmospheric 

 waves, it is proposed also, that hourly observations should be 

 made at all the stations for twenty-four hours, four times in the 

 year, namely, at the equinoxes and solstices. Occasional hourly 

 observations will likewise be made under special circumstances. 



" As respects the second head of inquiry, — namely, thephenomena 



