37. When a government servant is required by superior authority to travel by special means of conveyance the cost of which exceeds the travelling allowance which would otherwise be admissible, the actual cost of transit may be drawn instead of the travelling allowance. The bill must be supported by a certificate signed by the superior authority and countersigned by the controlling officer to the effect that the use of the special means of conveyance was necessary in the interest of Government work, and stating the circumstances which made it necessary.