MR. RONALDS ON PHOTOGRAPHIC SELF REGISTERING INSTRUMENTS. 113 
In pursuance of a hint or suggestion of the Astronomer Royal, a very useful addi- 
tion to the electrometer has been made, by which the hind in addition to the tension 
of the electrical charge is registered. It depends upon the same principle as that of 
the “ Dry Pile distinguishing apparatus,” and will be clearly understood by reference 
to the diagram. 
The above-described instrument, with its various improvements, made during the 
progress of the experiments, was used in connection with an atmospheric conductor 
situated much lower, and otherwise much less advantageously than our ordinary 
Kew-conductor*. 
In order to adapt the greater part of the apparatus to the purposes of a thermo- 
graph, a thermometer of the horizontal kind, and having a flat bore, is employed. 
Its tube is introduced through a side of the microscope in lieu of the electrometer ; 
a diaphragm is fixed upon it of narrower dimensions than the breadth of the mercury ; 
and the slit in the screen at the eye-end of the microscope is (of course) straight and 
horizontal ; the manipulation and all else remaining as before. 
The photo-harometrograph requires a somewhat different arrangement of the same 
microscope, &c. 
The long case in which the frame carrying the photographic paper slides, is now 
placed in a horizontal position. The clock with its pulley, &c. is fixed near to one 
end of that case : the usual line, attached to the pulley on the clock-arbor, enters the 
case, as before, and is hooked to the sliding frame, and provision is made of the 
same kind as before, &c. for ensuring its steady and regular motion. 
The lower leg of a siphon barometer is introduced through the non ; lower side of 
the microscope (in lieu of the thermometric tube), and a very light blackened pith- 
ball rests upon the surface of the mercury. In order to procure a clean and correct 
boundary-line in the photograph, a kind of contracting and expanding diaphragm on 
the barometer tube became very useful in this case, for when daylight was used, very 
minute adjustments of the aperture were required. The slit in the screen is (of 
course) vertical. The curves seen in the photographs represent the actual variations 
nearly of this siphon barometer, but it is my intention to try to fill a barometer of 
the cistern kind, in the late Professor Daniell’s manner, with a pith-ball on the surface 
of the mercury still, and then to use a magnified range of the image. 
The curve or right line which forms the right-hand boundary of the dark band, 
seen in the photographs of the thermometer and barometer, represents the variations 
of the mercury as accurately as the usual scale readings of ordinary instruments. 
The applicability of this system of self-registration to a magnetograph was suffi- 
ciently obvious; but a much more solid kind and disposition of apparatus is necessary. 
The two-feet magnet, now used for this purpose at the Kew Observatory, was 
kindly lent to me by the Astronomer Royal (in February 1846), and is suspended by 
* The photographs executed between July 24, 1845, and August 29, 1846, by means of this and my various 
other instruments, were produced at the meeting of the Royal Society, at which this communication was read. 
Q 
MDCCCXLV1I. 
