Before a really efficient burner could be produced, there were, however, to be successfully encountered other problems, the precise nature of which was not so clearly apparent as that of the one above referred to; otherwise their solution would not have been so long delayed. Of these, the most important, and at the same time the most difficult, were two--namely, the right adjustment of the air supply, and the most advantageous pressure at which to consume the gas. In the earliest Argands, not the slightest provision was made for diminishing the pressure of the gas before it was consumed. It was thought that everything had been accomplished that was necessary if the holes for its emission were sufficiently minute to allow of no more than the required quantity of gas passing through them at the extreme pressure at which it was supplied to the burner. The consequence was that the gas, issuing from the burner at a very high velocity, became so intermingled with air before it was consumed, that its flame was excessively cooled; and only a small fraction of the illuminating power available was developed. Then as to the air supply. In nearly every burner produced prior to Mr. W. Sugg's invention of the "London" Argand in 1868, this was greatly in excess of the requirements; nor is it to be wondered at. Had the supply of air been delicately adjusted, while yet there was no provision for diminishing the pressure of gas at the burner, the flame would have been liable to smoke on any sudden increase in the pressure of gas in the mains; and the annoyance and inconvenience occasioned by a smoking flame were greater drawbacks than the loss of light experienced through having the air supply greatly in excess. Thus, although during this period there were many so-called "improved" burners brought into notice, in none of them were these two cardinal requirements in the production of an efficient burner clearly recognized and seriously grappled with; and, consequently, the high level of excellence to which the Argand is capable of being brought was not attained. SUGG'S ARGANDS. [Sidenote: The 'London' Argand.]