How can you Substitue cream of tartar in meringues?

I'm not positive. The reverse substitution is for 1 tsp. of baking powder, you use 1/4-1/2 tsp. of baking soda and 1/2 tsp. cream of tartar. So you could try using baking powder instead of cream of tartar, but it's not guaranteed. If there is supposed to be baking soda and cream of tartar in the recipe and you substituted baking powder, that would be more likely to succeed.