This is a fascinating, relatively rare, cover(using a unit envelope!) sent from a soldier(likely officer, Captain G. Carew) of the 7th Hussars, on September 27, 1891! It was addressed to a Mrs. Russell Carew. The mail arrived in Watford(pretty nice strike!), England, on October 19, 1891!  

To continue, the green, 2.5-Anna GB British India stamp is in very fine condition! The squared-circle CDS from the Tinnevelly Railway Station P.O. is quite fine, but is somewhat light/faded in some areas. The transit cancel(10/1/91) from the Bombay P.O. is superb!  

Lastly, as for the envelope, itself, it's in pretty good condition considering its age and how much it has travelled! It's pretty clean. There are some tears along the top and bottom of the front, and some in the back flap connected to how the mail was originally, somewhat-roughly, opened. The back flap is about 66% complete. Noted blemishes aside, this is a really neat, uncommon piece of early 20th Century, British military history connected to the 7th Hussars!