In December 1915 additional P-class Zeppelins and the first of the new Q-class airships were delivered. The Q-class was an enlargement of the P-class, lengthened to , adding two gasbags, and improving both ceiling and bomb-load. Improved defensive measures made raids more hazardous, and several airships were destroyed. By mid-1916, there were 271 anti-aircraft guns and 258 searchlights across Britain, and the introduction of an effective combination of explosive and incendiary bullets gave the defending aircraft their first successes. New types of Zeppelin with improved ceilings restored the advantage, but led to further flying and navigation problems; oxygen was needed to fly at high altitude, the extreme cold led to crew fatigue and technical problems, and the meteorologists of the time did not appreciate the differing wind conditions likely to be met at altitude. Nevertheless, in 1916 twenty-three raids dropped 125 tons of bombs, killing 293 and injuring 691 people.