1873.] JiOL [Chase. 



being rather to get at the truth, than to build up or overturn auy plausi- 

 ble theory. 



On some accounts it would be desirable to have observing stations as 

 thickly scattered over our whole domain, as they are in England and 

 some other European countries. But on the whole, I think, that at least 

 for the time being, the gain is greater than the loss in having such dif- 

 fusion of reports as will facilitate the early discovery of practical general 

 laws, the influence of which might be obscured by the perplexities of 

 local disturbances, if the stations were twice as numerous. A great part 

 of the success of American meteorology, to which some of our foreign 

 competitors have given the palm with almost envious reluctance, may 

 have sprung from the broad generalizations which were at first forced 

 upon the Meteorological Committees of the American Philosophical So- 

 ciety and the Franklin Institute, by the magnificent breadth of the Mis- 

 sissippi Valley, and the consequent unbroken sweep afforded to the winds 

 that break over the ridges of the Rocky Mountains, or pour from the 

 Arctic Regions through the Valley of the Saskatchewan and across 

 the great lakes. The time will, however, soon come, if it has not already 

 come, when the need of more minute details will become evident, and 

 when State Legislatures, Scientific bodies, or local Boards of Trade, 

 should devise means for supplementing, or use their influence for widely 

 enlarging, the magnificent work of the National Bureau. 



The plan of recording observations at all the stations at the same 

 actual time, is undoubtedly the best for determining the mechanical forces 

 which are operating for producing meteorologic changes, and consequently 

 for the general purposes of weather forecast. It is, however, attended 

 with some inconveniences, especially in regard to temperature, the esti- 

 mated direction of isabnormal thermograms being liable to error on 

 account of inadequate allowances for differences of local time. The im- 

 portance of those thermograms may be readily understood. The baro- 

 grams determine .the gravitating influences of the moment, and the forces 

 that are now operating to effect the changes of a few succeeding hours ; 

 but if we wish to estimate the probabilities for a day or more in advance, 

 we must regard the probable changes in the barograms themselves. Those 

 changes are mainly dependent, either directly or indirectly, upon changes 

 of temperature, the direct influence being manifested by increase or 

 diminution of atmospheric density ; the indirect, by condensation of 

 moisture, in the form of rain or snow, and all tlie other attendant 

 phenomena. Each centre, line, or area of precipitation, becomes a pivot 

 or link between the upper and lower aerial currents, since there must be 

 a downthrust, as well as an indraught, to supply the place of the con- 

 densed vapor. The degree and the rapidity with which the lower will be 

 affected by the upper currents, will depend upon the number, position, 

 and extent of storm areas; and a knowledge of the difference of temper- 

 ature from the normal temperature for the month at each station, would 



