Levels or Drift mines were driven into the hillsides at an angle to reach the seams or veins of coal. In many cases, the method of working these small mines has altered little since the early miners first mined for coal in this way. Apart from the use of compressed air for blast picks to replace the mandrels and small picks, along with safety helmets and battery lighting which replaced candles and carbide lamps, very little has changed. The method of timbering to line underground roadways is also much the same as it was in the early days. Until the end of the 20th-century horses were still being used to pull the laden trams out of the small mines. In 1931 there were 73,024 horses in British deep and shallow mines but were reduced to 32,109 by 1937 with the increasing increase of machine power. Even though mechanical rope haulage engines were replacing horses, the coal still had to be removed from the coal face to the mechanical haulage, so for many years to come the horse played a large part in coal mining. Horses, as a rule, were very well treated, well-fed and generally not overworked. At any one time at Tirpentwys colliery in Gwent in excess of 100 horses were used and some were shown in Pontypool park in competitions.  The last two horses to work in a British coal mine was (at Pant-y-gasseg near Pontypool in Gwent ) and were named Gremlin and Robbie. Both were retired in  1999, with Gremlin going to Milton Keynes R.S.P.C.A, the home for retired pit ponies, while Robbie went to an English mining museum. Gremlin can be seen having worked his last shift and pulled his last tram of coal. Mike Desmond ( deceased ) was leading him out from the mine for the last time in the photograph below. Mike's family had been involved with small mines for over 70 years, and at the time he said that it bought a tear to his eyes seeing both horses going,  As can be seen those feelings were reflected in Mike's body language.