Section 21. In the service of precepts in criminal cases, the officer shall be allowed the actual, reasonable and necessary expenses incurred in going or returning with the prisoner, and if he necessarily uses his own conveyance, he shall be allowed therefor twenty cents a mile for the distance traveled with the prisoner, except that in the service of such precepts of the district court of Chelsea, if he necessarily uses his own conveyance, he shall be allowed, if the distance traveled is less than ten miles, thirty cents a mile for the distance traveled, both ways; and if he uses the conveyance of another person, he shall be allowed the amount actually expended by him therefor; but no allowance for the use of a conveyance shall be made unless the officer certifies that it was necessary for him to use a conveyance and that he actually used it for the distance, and, if the conveyance of another was used, that he paid therefor the amount, stated in his certificate. If, in the service of a mittimus, the journey from the town where the prisoner is held to the town where he is to be committed can be made by railroad, no allowance shall be made for the use of any other conveyance, unless the court from which the mittimus is issued by general or special order has authorized the use thereof.