The rifle was standing up against Mrs.Ballantyne's writing-table in a corner of the tent, when the doctor from Ajmere discovered it.

In the second place, although Ballantyne was found in the open, there was a patch of blood upon the carpet within the tent and a trail of blood from that spot to the door.

There could be no doubt that Ballantyne was killed inside.
There was the third point to establish that theory.

Neither the sentry on guard nor any one of the servants sleeping in the adjacent tents had heard the crack of the rifle.

It would not be loud in any case, but if the weapon had been fired in the open it would have been sufficiently sharp and clear to attract the attention of the men on guard.