The 30 cats in the trial were subjected to a total of 54 attempts to stop purring , proximity to a running tap caused 17 of 21 ( 81 % ) cats to stop purring , blowing at the cat 's ears worked in 2 of 15 ( 13 % ) cats , spraying an aerosol close to the cat was effective in 9 of 18 ( 50 % ) cases .