One of the most valuable services rendered is that of the warnings of cyclonic storms for the benefit of marine interests. These are displayed at nearly three hundred points on the ocean and lake coasts, including all important ports and harbors, warnings of coming storms being received from twelve to twenty-four hours in advance. The result has been the saving of vast amounts of maritime property, estimated at many millions of dollars yearly. Agriculturists also derive great advantage from these warnings, especially those engaged in the production of fruits, vegetables and other market garden products. Warnings of frosts and of freezing weather have enabled the growers of such products to protect and save large quantities of valuable plants. It is said that on a single night in a small district in Florida, fruits and vegetables were thus saved to the amount of more than $100,000. In addition, live stock of great value has been saved by warnings a week in advance of the coming of a flood in the Mississippi; railroad companies take advantage of the forecast for the preservation, in their shipping business, of products likely to be injured by extremes of heat or cold, and in various other ways the forecasts are of commercial or other value. One of the chief stations for observations is that at Mount Weather, in the Blue Ridge Mountains of Virginia. This is equipped with delicate instruments in considerable variety for the study of varying conditions of the upper air. Kites and captive balloons are sent up every favorable day, ascending to heights of two or three miles, and equipped with self-registering instruments to record the temperature and other conditions of the atmosphere. At other times, free balloons are liberated, carrying sets of automatic registering instruments. Some of these travel hundreds of miles, but nearly all are eventually found and returned. How does a Siren Fog Horn Blow? There are a great many different kinds of signals for the guidance of vessels during fogs, when lights or other visible signals cannot be perceived. One of the most powerful signals is the siren fog horn, the sound of which is produced by means of a disk perforated by radial slits made to rotate in front of a fixed disk exactly similar, a long iron trumpet forming part of the apparatus. The disks may each contain say twelve slits, and the moving disk may revolve 2,800 times a minute; in each revolution there are of course twelve coincidences between the slits in the two disks; through the openings thus made steam or air at a high pressure is caused to pass, so that there are actually 33,600 puffs of steam or compressed air every minute. This causes a sound of very great power, which the trumpet collects and compresses, and the blast goes out as a sort of sound beam in the direction required. Under favorable circumstances this instrument can be heard from twenty to thirty miles out at sea.