[6] To investigate rules for this purpose, 1. Let M represent the mould candle, _a_ its distance from the wall, on which the shadows were compared, _x_ its quantity of matter consumed in a given time, (_t_) and Q the quantity of light emitted by M in the same time: 2. Let _m_ represent any other candle, _b_ its distance from the same wall, and _y_ its quantity of matter consumed, in the time _t_. Then as the intensities of light are directly as the squares of the distances of the two candles from the wall, we have as _a_² : Q ∷ _b_² : (_b_² + Q)/_a_² = the quantity of light, emitted by _m_ in the time. Then let us suppose that the quantities of light are directly as the quantities of matter consumed in the time _t_, and we have, As _x_ : Q ∷ _y_ : (_y_ + Q)/_x_ = the quantity of light emitted by _m_ in that time, by hypothesis. Now, when (_b_² + Q)/_a_² (Theo. 1.) is = (Y + Q)/X (Theo. 2.) the quantities of light of M and _m_ are directly as their quantities of matter consumed in any given time. METHOD OF INCREASING THE LIGHT OF TALLOW CANDLES, AND TO OBVIATE THE NECESSITY OF SNUFFING THEM. Mr. EZEKIEL WALKER has shewn that, if a trifling alteration be made in the method of using common tallow candles, they will become excellent substitutes for those of wax. A common candle, weighing one-tenth of a pound, containing fourteen single threads of fine cotton, placed so as to form an angle of 30 degrees[7] with the perpendicular, and lighted, requires no snuffing; and what is much more valuable for some purposes, it gives a light that is nearly uniform in strength without the least smoke. These effects are thus produced: [7] Candlesticks may be made to hold the candle at this angle, or they may be so contrived as to hold the candle at any angle at pleasure. When a candle burns in an inclined position, most part of the flame rises perpendicularly from the upper side of the wick, and when viewed in a certain direction, it appears in the form of an obtuse angled triangle. And as the end of the wick projects beyond the flame at the obtuse angle, it meets with the air, and is completely burnt to ashes: hence it is rendered incapable of acting as a conductor to carry off part of the combustible matter in the form of smoke. By this spontaneous mode of snuffing, that part of the wick which is acted upon by the flame continues of the same length, and the flame itself very nearly of the same strength and magnitude[8]. [8] The wick’s not being uniformly twisted throughout, may occasion a little variation in the dimensions of the flame.