Mr. R. Potter on a New and Simjde Heliostat. 7 



rent daily motion of the sun being nearly in a circle round 

 the earth's axis, and sensibly also in a circle round the spin- 

 dle e o, if the mirror be so set as to reflect the sun's light in 

 the direction op 9 still parallel to the earth's axis, and if the 

 spindle revolve, with the mirror, once round in twenty-four 

 hours, then whilst the sun continues to shine on the mirror, its 

 light will be reflected in the same direction op. The truth 

 of this will be easily seen on considering that the positions of 

 the sun and the mirror must be the same at all times of the 

 day with respect to the line op; and to an eye placed at p 9 

 the reflected image of the sun would appear to stand still, and 

 hence the propriety of the appellation Heliostat. 



For the source of motion to the spindle, I have used a com- 

 mon clock, shown at fg, fig. 3, and fg h, fig. 2 ; the whole 

 apparatus being attached to a rectangular board, as seen in 

 the figures. The hands of the clock being removed, a grooved 

 pulley is fixed on the arbor, which had carried the hour-hand, 

 as at *, fig. 3. This pulley revolving once round in twelve 

 hours when the clock is going, communicates motion by means 

 of a band to the pulley k, fixed on the spindle e o, which be- 

 ing twice the diameter of the pulley i, causes the spindle to 

 revolve, as required, once in twenty-four hours. 



The band which I have found to answer well for the pulleys 

 is a strong cotton thread passed several times round them, and 

 then fastened with a loop, which leaves the means of adjusting 

 the band to a proper tightness. The pendulum should be 

 adjusted by the revolution of the spindle after the instrument 

 is complete, by which means any slipping of the band, or 

 want of accuracy in the dimensions of the pulleys, may be com- 

 pensated. 



Considerable care is requisite to keep this, as well as every 

 other instrument of a similar kind, correctly in position ; for 

 it will readily be perceived that the correct action of it de- 

 pends as much upon the spindle eo being accurately placed, 

 as upon the proper rate of the clock. I have had mine so 

 nearly adjusted, as to reflect the sun's light upon the same spot 

 on the ceiling of a lofty room so exactly, that no perceptible 

 motion could be detected during an hour and a half. 



It will be seen that neither this nor'Dr. S'Gravesande's heli- 

 ostat has any provision for counteracting the variation of the 

 sun's declination during the time of use, and of course the 

 instruments may be brought to act more correctly when the 

 sun is near the summer or winter solstice than at other times 

 of the year. 



If an instrument-maker were employed to construct this 

 heliostat, he would easily contrive the plan so as to connect 



