=Cirro-cumulus= are flocks of small detached fleecy clouds, generally white and without shadows. =Alto-stratus= is a grey or bluish veil through which the sun and moon are faintly visible, occasionally giving rise to coronÊ. Its altitude is only about half that of Cirro-stratus. =Alto-cumulus= are flocks of larger, more or less rounded, white or partially shaded masses, often touching one another, and frequently arranged in lines in one or more directions. =Strato-cumulus= are large globular masses or rolls of dark cloud, frequently covering the whole sky, especially in winter. =Cumulus= are piled clouds with conical or hemispherical tops and flat bases. They are formed by rising currents of heated air, and are therefore most common in summer and in tropical regions. When broken up by strong winds the detached portions are called Fracto-cumulus. =Cumulo-nimbus= is the massive thunder shower cloud rising in the form of mountains or turrets, and generally having above a screen of fibrous appearance (False Cirrus), and underneath a mass of cloud similar to Nimbus from which rain falls. =Nimbus= is a dense, dark sheet of ragged cloud from which continued rain or snow generally falls. Broken clouds underneath, forming the scud of the sailors, are called Fracto-nimbus. =Stratus= is a thin uniform layer of cloud at a very low level. When the sheet is broken up into irregular shreds it is called Fracto-stratus. Having described the origin and appearance of the different clouds, an account will now be given of the measurements made at Blue Hill Observatory and the information which they give about the circulation of the atmosphere. The work there was taken up in 1887 in consequence of the interest of the meteorologist, Mr. Clayton, in the study of clouds; his discussion of the cloud observations, published two years ago with the Blue Hill observations, has been termed by far the most thorough study of the kind ever undertaken in America if not in the world. Most of the conclusions which are stated popularly here have their scientific expression in his work.