The 150 foot solar observatory tower was used for fire detection duty during 1919 and the 1920's. Here are examples of stories published at the time:

May 13, 1919:  "Details have been perfected for the regular patrolling of the Los Angeles National Forest in Southern California beginning on June 1.  Officers of the Army Balloon School at Arcadia and March field near Riverside have mapped out the scheme which is believed to be the first time in history of aeronautics that the plan has been tried.  Methods of patrolling, observation, giving fire warnings, and other particulars are set forth in the orders which follow: 

An observation balloon will be stationed at an elevation of about 3000 feet at the Balloon School Field near Arcadia.  This observer will be on duty from 7 a.m. to 2:30 p.m. each day.  He will be furnished with a map of the front of the Los Angeles National Forest from La Canada to San Dimas Canyon and can probably give adequate lookout service as far north as the crest of the San Gabriel watershed.  The student detachment learning observation now stationed at Mt. Wilson will also render lookout service.  Reports of fires from both the balloon observer and the Mt. Wilson detachment will come in by phone and the Balloon School at Arcadia and will be transmitted to the Forest Service at Los Angeles, who will call upon the appropriate unit of suppressing force."  (The Bend Bulletin - Oregon)

July 15, 1924:  "A forest fire broke out this afternoon on the west side of San Gabriel canyon, above Opid's camp, according to word telephoned here by forest rangers in the district.  The fire was reported to be gaining headway.  No further details have been received.
     The fire was first observed by a lookout at the Mount Wilson observatory, who telephoned the alarm to forestry officials here and in Los Angeles."  (Modesto Evening News)