228 HELIOTROPES. 



By this simple expedient, it would soon appear, that there 

 is no such thing-, strictly speaking, as a solstice j for, from the 

 shortest day, the owner would, every clear evening, see the 

 disc advancing, at its setting, to the westward of the object ; 

 and, from the longest day, observe the sun retiring backwards 

 every evening, at its setting, towards the object westward, till, 

 in a few nights, it would set quite behind it, and so by degrees 

 to the west of it ; for when the sun comes near the summer 

 solstice, the whole disc of it would at first set behind the object: 

 after a time, the northern limb would first appear, and so every 

 night gradually more, till at length the whole diameter would 

 set northward of it for about three nights ; but, on the middle 

 night of the three, sensibly more remote than the former or 

 following. When beginning its recess from the summer tropic, 

 it would continue more and more to be hidden every night, till 

 at length it would descend quite behind the object again ; and 

 so nightly more and more to the westward 



relation to the latitude in which the instruments were placed, or to the 

 degrees of solar influence that might exist in the regions in which they 

 were used, and of which they would partake ; he also noticed, in coin- 

 cidence with these movements, the daily expansion and contraction of 

 the petals and leaves of most plants, and that the different species of the 

 heliotropium and chrysanthemum, turned their corollse round toward the 

 sun for many hours during the day. Hence he concluded that an 

 instrument might be constructed upon principles nearly similar to the 

 laws which regulate these motions in plants. 



This instrument he formed of a circular ring of cork, three inches in 

 diameter. Into this is fixed twenty-five needles fully impregnated with 

 the magnetic fluid, and these are placed at equal distances round the 

 circumference of the circle, with their north and south poles placed out- 

 wards alternately. This circle is affixed to a light slip of wood, five inches 

 long, and one-fourth of an inch broad, by a piece of copper wire, of a 

 semicircular form, the extremities of which are passed through the 

 opposite sides of the cork's circle ; and the slip of wood attached to the 

 centre of the wire. Into the centre of the bar is fixed an agate cup ; arid 

 the whole traverses like a compass needle upon a fine steel point, the bar 

 of wood being equipoised by a small weight at the end of it, equivalent to 

 the weight of the needles. This instrument, when placed with a disc of 

 purple velvet across the needles, in the sun's rays, continued to revolve 

 nearly the whole day, moving always in the direction from east to west 

 by south, in the course of the sun's apparent motion. It moves forty or 

 fifty degrees to the light of a single candle held close to the side of the 

 circle. A piece of clear amber, formed into a convex lens, if fixed into a 

 circle of cork, and suspended by a fine hair or filament, under a glass 

 cover, will also be arrested by the incidence of the solar rays, and will 

 continue to present its surface to the sun, if unclouded, as long as he is 

 invisible above the horizon. 



It is, perhaps, not generally known, that the conducting power of living 

 plants, in favouring the rapid distribution of electricity, r-as been reckoned 

 three millions of times greater than that of water. ED. 



