The work done at Blue Hill shows the importance of cloud observations to elucidate the general movements of the atmosphere, as well as the circulation of the air above barometric maxima and minima, which can result practically in making accurate weather forecasts possible a day or two in advance. The systematic observation of the upper currents was brought to the attention of the International Meteorological Committee by Dr. Hildebrandsson in 1885, and at the meeting of the International Cloud Committee in 1894, besides the adoption of the nomenclature of clouds and instructions for observing them, it was decided that observations of their motion, as well as measurements of their height, should be made in various parts of the world. Accordingly, the year commencing May 1, 1896, was designated as the "International Cloud-Year," and observations with nephoscopes of the direction of motion and relative velocity of clouds were begun at many stations in Europe and Asia, and at fifteen stations in the United States. Trigonometrical measures of the heights of clouds were undertaken at stations in Norway and Sweden, Russia, Finland, Prussia, and France, as well as at Toronto, Manila, and Batavia; in the United States the measurements already described were recommenced at Blue Hill, and the Weather Bureau equipped a similar station in Washington. In Europe it is thought that the determination of heights by photogrammeters, as the theodolites with attached photographic cameras are called, possesses advantages over the visual theodolites, and it is true that not only is the kind of cloud recorded on the plates, but there are available for calculation as many points on the cloud as can be identified on the two plates exposed simultaneously at both stations. On the other hand, in the case of nearly uniform or dark cloud-strata, it is easier to see points for measurement on the cloud than to fix them on the photographic plates. For this reason, and from the difficulty of manipulating the photogrammeter, visual instruments were adopted both at Blue Hill and at Washington. The work was successfully carried on until May 1, 1897, and the observations and measurements were reduced at Blue Hill according to the plan prescribed by the Committee. Already the observations and measurements made at Upsala, Manila, and Blue Hill are published, and the others will follow. The discussion of the correlated data from the various countries will probably increase our knowledge of the circulation of the atmosphere, which is certainly one of the most interesting and important questions in the physics of the globe. The result will have been reached by international co-operation, of which the benefits to science are everywhere manifest to-day. But for the whole problem to be solved, it is necessary, not only to know the movement of the air, but, as far as possible, to ascertain its conditions of heat and moisture. This may be accomplished by the use of balloons and kites,