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rain. In searching for observations to test this conclusion, I found a decided diurnal inequality in the rain-fall at Philadel. phia showing a maximum about 6 P. M. and a minimum at 3 A. M. The observations made by the United States Signal Service did not show any decided diurnal inequality, owing, perhaps, to their including only three daily observations. In a series of hourly observations made at seven stations in Great Britain I found evidence of two daily maxima and two daily minima.
Since the publication of my former article I bave found in Kreil's Klimatologie von Böhmen the results of ten years' observations at Prag, lat. 50° 5', which show a decided diurnal in. equality, having a maximum about 4 P. M. and a minimum about 7 A. M., with a second maximum which is less distinctly marked. The following table shows the average annual rain-fall at Prag as deduced from observations from 1850 to 1859 expressed in Paris lines :
Rainfall at Prag, Austria.
These numbers follow a law bearing a close resemblance to that of the Philadelphia observations, and lead us to presume that a similar law must prevail in the fall of rain over a considerable portion of the United States.
I have received the hourly observations of rain-fall at seven stations of Great Britain complete for the year 1874. At most of the stations there are evidently two periods of maximum rain-fall, but the time of maximum appears to depend very much upon local circumstances.
Comparison of storm paths in America and Europe. In comparing the tracks followed by storms in America and Europe I have depended chiefly upon the following materials :
1. The daily United States Signal Service maps from 1871 to 1875, and especially the monthly maps showing the tracks of storm centers.
2. Atlas des mouvements generaux de l'atmosphere, redigé par l'observatoire Imperial de Paris, embracing 18 months, from June, 1864, to Dec. 1865.
3. Cartes synoptiques journalieres construites par N. Hoffmeyer, Copenhagen, embracing 9 months, from Dec. 1873 to Aug. 1874.
In order to determine what may be called the average track of storm centers in the United States, I ruled a large sheet of paper with several vertical columns headed 122°, 117°, 107°, etc., these numbers denoting degrees of longitude from Green: wich. I then took one of the monthly maps showing the tracks of storm centers, and following each of the tracks in succession determined in what latitude it crossed the meridians indicated at the top of the table, and the results were set down in the ap. propriate column. I proceeded in the same manner with each of the monthly maps and then took the average of all the numbers in each column. The results are shown in the first two columns of the following table; where column first shows the meridians of longitude from Greenwich, and column second shows the average latitude in which each of these meridians is intersected by the storm paths. The curve thus determined is traced on the accompanying chart and passes over the center of Lake Erie. It will be seen that the average direction of storm paths is not the same on all meridians. The directions given in my former article (see this Jour., vol. x, p. 1) must be understood to be the average direction of storm paths for the region covered by the United States observations; and this represents pretty nearly the mean direction for a place whose latitude is 421° and longitude 8370 W., which is nearly the position of Detroit, Michigan.
Average direction of storm-paths.
I proceeded in a somewhat similar manner with the Paris maps, but since these maps do not generally exbibit lines
drawn so as to show the tracks of storms from day to day, I placed in one column the latitudes of all the storm centers between the meridians of 60° and 45° W. from Paris ; in a second column I placed those between the meridians of 45° and 30° W., and so on for each 15° of longitude. I then took the average of all the latitudes in each of the columns. The result is shown in columns third and fourth of the preceding table, and the path thus determined is traced on the accompanying chart. This path passes over Dublin, and will be seen to form a natural continuation of the track deduced from the American observations. This result is doubtless accidental and is due to the fact that near the American coast the observations from which the Paris maps were constructed were derived from a region extending but little north of the stations of the United States Signal Service. If the Paris maps had included observations from Labrador and Greenland the average track of the storms represented would have been more northerly than it now is.
I proceeded in a similar manner with Hoffmeyer's charts except that the meridians were selected at intervals of 10°, and the results are shown in columns fifth and sixth. The path thus determined lies several degrees north of that previously determined, and this arises from the fact that the maps exhibit the results of observations made in Greenland and Iceland as well as from more southern latitudes. In order to deduce an average result from the French and Danish maps I have combined them in a single series, and the result is shown in columns seventh and eighth of the preceding table. The path thus determined is traced on the accompanying chart and passes through the northern extremity of Scotland.
In order to show the connection between storm paths and the mean height of the barometer, I bave drawn upon the same chart two other barometric lines. The mean height of the barometer at the level of the sea varies with the latitude of the place. On the Atlantic ocean at the equator the mean height of the barometer is about thirty inches. If from this point we travel northward the pressure increases, and in latitude 30° becomes about 30.2 inches. Thence the pressure diminishes to 29.6 inches near latitude 70°, from which point the pressure slightly increases as we advance north ward. A somewhat similar result takes place in going from the equator to the North Pole upon any meridian, but the maximum pressure is not the same under all meridians, and the same is true of the minimum pressure. The undulating line near the bottom of the accompanying chart shows the line of the greatest mean pressure varying on different meridians from about 30 inches to 30-2 inches. The undulating line near the top of the chart
latitude whence the and in latiing point
shows the line of the least mean pressure, being about 29.6 inches on the meridian of Greenwich and increasing somewhat as we proceed either east or west from that meridian. These lines are drawn chiefly from data collected by Alexander Buchan. (See Edinburgh Phil. Trans , vol. xxv.) .
We perceive then that at all places near the southern margin of the chart the mean pressure of the atmosphere is greater than it is further northward, and this is generally sufficient to cause an average surface wind from south to north although the wind advances from a warmer to a colder region. On the other hand, at places near the northern margin of the chart the mean pressure of the atmosphere is somewhat greater than it is further south, and this force combined with a lower mean temperature causes a surface wind from north to south. Here then are permanent causes producing winds from opposite directions near the upper and lower portions of the chart, and these must be a permanent source of storms independent of those inequalities of pressure which arise from causes of a more local nature.
The average path of storms in their progress from America to Europe is apparently modified by the line of greatest mean pressure. This line has a more northerly position in Europe than it has in America, and this may be the reason why storm tracks generally bend northward in advancing from America to Europe. There are some minor particulars in which storm paths are apparently modified by the line of greatest mean pressure; but instead of attaching importance to coincidences which may prove to be accidental, it is more prudent to wait and see if these peculiarities are confirmed by further observations.
Oscillations of the barometer in different latitudes. For the purpose of determining in what region of the globe the oscillations of the barometer are the greatest, I have prepared a table showing the mean monthly oscillation of the barometer at as many stations as possible in high northern latitudes. A few of the numbers in the following table are derived from Kaemtz Meteorology, edited by C. V. Walker, p. 297. The other numbers have been collected by myself from various sources which are indicated in the last column, and some of the results have required a careful discussion of many years' observations. Column fourth shows the average monthly range of the barometer for the three winter months, and column fifth shows the same for the three summer months expressed in English inches
As some of these numbers depend upon observations of only one year, and therefore do not represent mean values very ac
curately, I have endeavored to combine them so as to obtain a
Mean monthly oscillation of the barometer for winter and summer.
Monthly oscillation of the barometer (normal values).