Page:The museum, (Jackson, Marget Talbot, 1917).djvu/87

Rh, when the air needs to be heated there is no advantage in this more costly system, for the dampened air in passing over the hot coils is dried. Another system is usually installed providing either large pans of water, which are placed on the hot coils, or a very fine spray which rises with the warmed air into the galleries.

Another system passes the air through sheets of water arranged one behind another. By warming this water in winter the air is warmed and moistened and so rises to the galleries. Some such system is necessary in order to keep the humidity nearly the same all the year round. A temperature of sixty-five degrees to sixty-eight degrees is right for museum galleries. Curiously enough the Directors of Italian picture galleries find it impossible to heat their museums to the same degree that can be done in London without injury to their paintings, and the cause of this is undoubtedly the difference in dampness in the London climate over that in Italy.

Another very important fact that must be borne in mind is the necessity of keeping the temperature throughout the twenty-four hours somewhere near the same. Where European galleries have failed has usually been in providing heat only during the day. The drop in