from the water in which they have been plunged, as soon as the redness in the centre of the drop ceases to be visible.
Since the smallest portion of any polarizing crystal polarizes or depolarizes light according to its position, the author expected to find the same property in the fragments of a broken drop, but upon trial they did not appear to possess this property.
Of the many important conclusions to which the author thinks that these experiments are calculated to conduct us, there is one which he considers too palpable to be passed over, namely, that when the particles of glass are separated to a certain distance by the expansive agency of heat, they assume a crystalline arrangement, which would not be discovered but by fixing them in this state by sudden cooling; since the gradual approximation of the particles, by slow cooling, entirely destroys the crystalline structure thus produced.
In a note the author remarks, that on more than one authority steel is said to be less dense after being hardened by quenching than before, which he ascribes, as in glass, to the sudden induration having commenced at the surface. And he takes occasion to suggest the possibility, that under these circumstances moderate changes of temperature may not occasion any degree of expansion, and that we may obtain, within certain limits, a substance of invariable length that may be useful for pendulums.
The present instrument depends upon a new extension of the principle of the common sliding-rule; for as in that numbers themselves are multiplied or divided by the mechanical addition of their logarithms, so in this their logarithms are multiplied or divided by mechanical application of corresponding logometric spaces.
In the common tables of logarithms, that of 10 is 1, and those of its simple powers are 2, 3, 4, &c.; so also the logarithm of the square root of 10 is 12, or ·5; the fourth root is 14, or ·25, being a decimal index expressing a power of 10 less than unity. In the same manner all other numbers are considered as powers of 10, and their logarithms are integral or decimal indices of those powers.
In the common sliding-rules the divisions are so placed as to mark intervals that are proportional to these indices; so that by simple juxtaposition the sum or difference of any two indices, and consequently the product or quotient of any two numbers, appears by inspection.
In this manner, by addition of two equal logometric intervals, the square of any number may be found; but the instrument so constructed is not prepared to give the higher powers, without proportionally frequent repetitions of the same process, which gives at length a multiple of the index by the tedious operations of repeated addition.
