CONCLUSION.

The tables and diagrams of this section exhibit very clearly the principles which it is the object of this paper to elucidate, viz.: that to whatever latitude a polar ice-field may extend, precipitatior. can take place only along its outer margin; and that the temperature of its central portions must sink too low to sustain appreciable quantities of aqueous vapor. They corroborate the statements made in Part I in regard to the atmospheric conditions produced by any vast condenser.

SEC. III. EVIDENCE THAT THE FOREGOING CONCLUSIONS ARE LEGITIMATE AND REASONABLE.

1. HEAT-DISTRIBUTING AGENCIES ARE LEAST EFFICIENT WHEN THE ATMOSPHERE IS COLD AND dry.

Effect of diminution of temperature.-Professor W. H. Dove's invaluable work on the "Distribution of heat over the surface of the Earth "39 summarizes the results of a vast number of observations on terrestrial temperature, at many stations; and it is yet the standard authority on the subject. The "normal temperature" of each tenth parallel from the equator to N. lat. 70° was determined by taking the mean of a series of observations at thirty-six equidistant points on each; and from the figures so obtained, the temperatures at N. lat. 80° and at the pole, were found by construction.40 Table II (in Section II) exhibits the temperatures thus obtained. It shows that the mean range of temperature from equator to pole is 77.4° F.- from +79.7° to +2.3°.

39" Die Verbreitung der Wärme auf der Oberfläche der Erde. Mit 7 charten und 2 Temperatur-Tafeln." Berlin, 1852.

40 Auf den zwölf Charten der Monatsisothermen habe ich von 100 zu 100 Länge zwischen den Isothermen graphisch für die Parallelkreise 0. 10. 20. 30. 40. 50, 60. 65. 70 interpolirt und auf diese Weise für jeden der angeführten Parallelkreise die Temperatur von 36 gleich weit von einander abstehenden Punkten erhalten. Das Mittel aus diesen 36 Zahlen nenne ich die normale Temperatur des Parallels, d. h. die, welche er an allen Punkten zeigen würde, wenn die auf ihm wirklich vorhandene aber verschieden vertheilte Temperatur auf ihm gleichförmig vertheilt wäre. Für den Parallel 80° und den Pol wurde sie auf folgende Weise erhalten. Ich theilte die Polar-projection der Erde in vier gleichseitige sphärische Dreiecke und bestimmte innerhalb jedes derselben die mittlere Wärmeabnahme von 0° bis 70° Breite. Die Temperaturabnahme in den beiden einander gegenüberliegenden Dreiecken wurde nun als Ordinaten für eine Abscisse aufgetragen, welche der die beiden Dreiecke als Loth halbirende Meridian war. Der Pol lag in der Mitte dieser Abscisse. Anfang und Endpunkt denselben im Acquator. Dadurch erhielt ich für die beiden Dreieckspaare zwei concave Curven, welchen beiden das mittlere Stück (80°-90° 80°) fehlte, die sich aber der Natur der Sache nach in der mittlesten durch 90° gehende Ordinate schneiden mussten. Wählt

Now if the heat-distributing agencies are as efficient in equalizing the temperature of the globe in winter as in summer, the winter range of temperature from equator to pole will not be very much greater than the summer range. On the other hand, the considerations detailed in section I enable us to predict a considerably greater range in winter. The accompanying table VIII shows the mean January and July temperatures at each of the several parallels as determined by Dove,41 and also the thermometrical range (from equator to pole) for each of these months. The table is graphically depicted in diagram V, into which the mean annual temperature is also introduced for comparison.

As indicated by the physical laws already referred to, the thermometrical range is much greatest in winter, being in fact 2.2 times as great as in summer.

A part of this difference in the thermometrical range for the two man nun die Dreieckspaare so, dass die Temperaturabnahme in dem möglichst verschieden ist von dem in dem gegenüberliegenden Dreieck, so sind die unverbundenen Schenkel jeder der beiden Curven sehr verschieden gegen die Abscissenachse geneigt. Dies combinirt mit der Bedingung des Durchschneidens beider in der durch den Pol gehenden Ordinate, engt die Weit erführung der Curven in ziemlich enge Grenzen ein und man erhält auf diese Wiese die Temperaturen der fehlenden Breiten von 80° und 90°— Op. cit., p. 13.

41 Op. cit., p. 13.

seasons is undoubtedly due to the varying position of the hemisphere with relation to the sun. It is only a part however. To demonstrate this, table IX and diagram IV have been prepared. The first column in the table exhibits the mean July temperature of the northern hemisphere as ascertained by Dove, and the second the heat-units received from the sun on June 15th, as determined by Meech. The maxima and minima of the solar intensity and terrestrial temperature do not correspond in time, the latter being about a month later than the former. The two columns are therefore practically equivalent. The third column represents corresponding absolute temperature in degrees Fahrenheit, calculated as in table VII, and of course, subject to the same limitation. The fourth column exhibits the difference between observed and calculated temperatures, and pari passu, the effect -absolute and relative-of the heat-distributing agencies in equalizing the temperature of the northern hemisphere in summer. The remaining columns exhibit similar values for December 13th (computed) and January (observed).

TABLE IX.

Observed and computed July and January Temperatures of the Northern Hemisphere.

43 Temperature of space, as determined by Herschel and Pouillet. 43 The means are arithmetical.

-108.3

-137.3

In diagram IV, this table is graphically represented (by the heavier lines)—the vertical scale being only one-half that employed in the other diagrams. The most widely separated curves will be seen to resemble those drawn by Meech44 to represent the actual

heat-distributing agencies having been overlooked by him. A

44 Plate IV of the memoir already cited. The ordinates and abscissæ have not, ho ever, the same relative proportion as in diagram IV.

glance at the diagram will show that the solar intensity and terrestrial temperature are far from proportional.

Now, if the efficiency of the heat-distributing agencies were constant in spite of fluctuations in temperature and humidity, we ought to find the difference between observed and computed temperatures the same in the two opposite seasons. In direct oppo

sition to this, but in accordance with the physical laws heretofore alluded to, we find the difference to be much greater in winter than in summer; the means being as 90.9 to 137.3, indicating that the equalizing agencies are more than 1.5 times as effective in summer as in winter. The relative areas enclosed by the upper and lower pairs of curves in the diagram illustrate this relation very clearly. In this case all vicious perturbations due to the inclination of the terrestrial axis to the plane of the ecliptic are, of course, eliminated.

As shown by the table, diminishing the mean temperature of the northern hemisphere 33° (from 62° to 29°)45 reduces the efficiency of the temperature-equalizing agencies by one-third. Now assuming the efficiency to be reduced in the same ratio by any further diminution of temperature, it is manifest that when the mean temperature falls to-38° (or 100° below +62°), the efficiency of these agencies will be wholly destroyed; yet this is but little more than half of the diminution that would take place, according to table IV, over an extensive polar ice-sheet. Moreover, the ratio is probably not a direct but a decreasing one, as the vapor-tension (which is an index of the temperature-equalizing capacity, as will be shown in the following subtitle) decreases much more rapidly than the atmospheric temperature, as exhibited in the first table and diagram of Section I.

Effect of atmospheric dryness.-The largest and most elaborate of Professor Dove's charts (Charte III) exhibits the monthly isotherms for January and July on the same sheet. A glance at the chart shows that these lines approach most closely over humid regions, and diverge most widely over arid areas, as in central Asia and in western-central North America. The same tendency is equally manifested in the chart devoted to the representation of "Isothermen von 4 Grad u. 20 Grad in ihrer jährlichen Wanderung."

45 These are simple arithmetical means and are lower than the true means in about the ratio of 46:58.