mospheric pressures are chiefly distributed over the northern hemisphere in January, and the lowest in July. If the pressure due to the elasticity of the vapour of the atmosphere was deducted from that of the whole barometric pressure, the pressure of the dry air which remains would present this relation in a much stronger light. The causes by which the dry air should have a minimum pressure in the hottest months of the year have been already referred to in par. 96. 683. ISOBAROMETRIC CHARTS of dry air for the globe, taken in connection with charts representing the whole pressure, may be regarded as the last step to be taken towards attaining a position whence the great movements of the atmosphere over the globe may be observed, and the causes of these movements satisfactorily explained. But we are very far from this result, owing to the scantiness of observations, and reduced means, of the elastic force of atmospheric vapour. Considering the great importance of the inquiry, it is hoped that meteorologists in different parts of the world will lend their assistance, by reducing and publishing their observations of the vapour of the atmosphere. 684. Whilst the rule is, that the minimum pressure of dry air occurs in the hottest months, there are important exceptions. The following table gives the mean pressures of the dry air at five places in the British Islands from Sandwick in Orkney to Greenwich, calculated for the ten years from 1857 to 1866; they are not reduced to sea-level : Sandwick. Culloden. Aberdeen. Glasgow. Greenwich. January,.. Inches. Inches. Inches. Inches. Inches. Hence in the north of Scotland the pressure of dry air is greater in summer than in winter; at Aberdeen nearly equal; but at Glasgow the pressure of the dry air is greater in winter than in summer, and at Greenwich the difference in favour of winter is still greater. It does not admit of a doubt, that in Iceland the pressure of dry air in summer exceeds that in winter in a still greater degree than in Orkney. Deducting the mean elastic force of vapour, as observed in Iceland during the three winter and summer months of 1866 and 1867, from the whole barometric pressure of thirteen years, it is seen that the probable pressure of the dry air in winter is 29.215 inches, whereas in summer it is 29.402 inches. At Sitka, in the north-west of North America, the pressure of the dry air on a mean of ten years is 29.393 inches in January, and 29.515 inches in July; whereas to the south, at New Westminster, in British Columbia, the pressure of the dry air for two years was 29.885 inches in January and 29.596 inches in July-thus showing the same peculiarity to prevail in the North Pacific that prevails in the North Atlantic. 685. This low winter pressure is explained by the ascent, and thence flowing away, through the upper currents, of the moist warm air which is poured into these regions from the respective oceans to the south of them; and the outflow is maintained at the high rate necessary to reduce the winter pressure to so marked an extent by the extraordinarily high pressures in Asia, Europe, and North America, by which they are surrounded. As the pressure falls in summer over these continents, the force, arising from the difference of the pressures, which originates and maintains the outflow, is diminished; and consequently less air being drawn off from the North Pacific and North Atlantic in summer, the pressure is more nearly maintained at a normal height. This is doubtless the explanation of the higher pressure of the dry air in these regions in summer-viz., a falling-off of the outflow owing to a diminished pressure all around, and not, as has been supposed, to an overflow into these regions by the upper currents from regions contiguous. 686. It is evident that very great differences between the winter and summer pressures of dry air will take place in dry climates, where the annual diminution of pressure which occurs arises solely from the high summer temperature. It is to the centre of Asia we must look for an example of this; and accordingly the pressure of the dry air at Barnaul, in Siberia, on a mean of ten years, is 29.760 inches in January, and 28.666 inches in July,-thus showing the enormous difference of nearly eleven tenths of an inch. 687. This question has important bearings on storms, particularly on the storms of Great Britain, which is situated within the southern limits of the region of most anomalous barometric pressure, which has its centre in Iceland. As the summer temperature falls in September and October the pressure is increased over the continents of Asia, Europe, and North America, and consequently air-currents are poured into the northern part of the Atlantic, in larger volume and with accelerated velocity; and since at the same time the temperature is everywhere falling, the whole atmosphere becomes saturated with vapour. This accumulated vapour is discharged in those furious storms of wind and rain which strew our shores with wrecks, and carry misery and desolation into the hearths of our seafaring population. Considering the important interests at stake, no amount of labour and no expense can be regarded as too great which might lead to a better understanding of these great atmospheric disturbances. .052 .053 .054 .055 .054 .056 .059 .088 .090 .091 .093 .090 .092 .094 .096 .095 .097 .098 .098 .099 .101 .100 .101 .103 .108 .100 .102 .103 .105 .117 .119 .129 .126 .146 .149 .151 .141 .143 TABLE II.-FOR CONVERTING MILLIMETRES INTO ENGLISH INCHES AND DECIMALS. 661 26.024 687 27.048 713 28.071 739 29.095 765 30.119 662 26.063 688 27.087 714 28.111 740 663 26.103 689 27.126 715 28.150 741 664 26.142 690 27.166 716 28.189 742 665 26.182 691 27.205 717 28.229 743 666 26.221 692 27.245 718 667 26.260 693 27.284 719 668 26.300 694 27.323 720 669 26.339 695 27.363721 670 26.378 696 27.402 722 671 26.418 697 27.441 723 672 26.457 698 27.481 724 673 26.497 699 27.520 725 674 26.536 700 27.560 726 675 26.575701 27.599 727 28.623 753 29.646 779 30.670 676 26.615 702 27.638 728 28.662 754 29.686 780 30.709 677 26.654 703 27.678 729 28.701 755 29.725 781 30.749 678 26.693 704 27.717 730 28.741 756 29.764 782 30.788 679 26.733 705 27.756 731 28.780 757 29.804 783 30.827 29.134 766 30.158 28.268 744 29.174 767 30.197 29.213 768 30.237 29.252 769 30.276 29.292 770 30.316 29.331 771 30.355 28.347 746 29.371 772 30.394 28.386 747 29.410 773 30.434 28.426 748 29.449 774 30.473 28.465 749 29.489 775 30.512 28.504 750 29.528 776 30.552 28.544 751 29.567 777 30.591 28.583 752 29.607 778 30.630 680 26.772 706 27.796 732 28.819758 29.843 784 30.867 681 26.812 707 27.835733 28.859 759 29.882 785 30.906 682 26.851 708 27.875 734 28.898 760 29.922 786 30.945 683 26.890 709 27.914 735 28.938 761 29.961 787 30.985 684 26.930 710 27.953 736 28.977 762 30.001 788 31.024 685 26.969 711 27.993 737 29.016763 30.040 789 31.064 686 27.008 712 28.032 738 29.056 764 30.079 790 31.103 TENTHS OF A MILLIMETRE IN THE DECIMAL OF AN INCH. 0.000 0.004 0.008 0.012 0.016 0.020 | 0.024 0.028 0.031 0.035 |