We have received a letter from Mr. J. W. Hoard, of Providence, R. I., in reference to the Aove subject, alluding to the article on page 313 of the present volume of the SCIENTIFIC AMERICAS . lie has made quite a number of experiments to prove -whether the best method of regulating the flow of gas to tk burners is by the valves at the burners, or the main valve near the meter. The results of these have convinced him that there is a difference of 10 per cent in favor of regulating by the burner valves, leaving the cock near the meter full open. This is a different opinion from that expressed on the page referred to, and which was obtained from one who has devoted much attention to the subject. This is a matter with which Mr. Hoard is well acquainted, as he has long directed the energies of his mind to the inventing of apparatus for regulating the flow of gas to burners.