majority of the nights of a year, and many nights yield "seeing" that might be considered perfect. A glance at the illustrations showing the mountain as seen from the east and from the west will make it evident at once why these conditions obtain. With the exception of a saddle running eastward, the land slopes away rapidly from the summit down into deep valleys, so that there is but little opportunity for heat waves radiated from surrounding land to mount to the atmosphere above the observatory and create atmospheric disturbances. The mountain is not so very high (4,209 feet above mean sea level), but it is high enough to hold the observatory in an atmosphere free from dust, smoke and fog. Being near the ocean, fogs are very frequent at certain seasons over the valleys in this region. It is seldom, however, that they mount high enough to envelop the observatory. Many evenings and early mornings fog completely fills the surrounding valleys, so that the observatory seems to rest on an island in a vast sea of fog. Often peaks only a few hundred feet lower than Mount Hamilton are covered by the fog, yet the work with the great instruments is uninterrupted. The picture "Fog in the Valleys at Sunset" gives a better idea of this condition than I can describe. In such a location as this the 36-inch refractor can be used with its maximum power a large portion of the time. In less favorable localities even larger instruments would not be so efficient.
It is one thing to have an excellent plant, and it is another thing to have men skillful enough to operate such a plant effectively. A very proficient marksman can not do very much damage with a blunderbuss,
