ON THE OBSERVATORY AT KEW. lll 
graphs. These different pieces fit so as to enclose the’ magnet hermetically, 
and thus the air can be rarefied or withdrawn by means of an air-pump in 
communication with a tube which passes through the marble disk and opens 
into the enclosure. This exhaustion of the air prevents the influence upon 
the magnets of currents of air. 
_ “Three telescopes, directed to the mirrors of the magnetographs, are 
established on two stone pillars, and have each an ivory scale the divisions of 
which are reflected, by the moveable and by the fixed mirror, into the interior 
of the telescope, offering in the field of view two very distinct images of the 
scale, one of which moves with the mirror of the magnet, so that at different 
times different divisions of this scale will appear to coincide with the vertical 
wire of the telescope. By the comparison of these divisions with that of the 
image which is fixed, the position of the magnet at any moment may be 
known ; so that, besides the continuous photographic record going on out of 
sight, and only taken account of every other day, there may be obtained, on 
any occasion, direct observations, which is a consideration of great importance. 
For example, if there is a magnetic disturbance, not only ean it be observed 
at the instant of its occurrence, but also direct observations may be obtained 
of oscillations which by their amplitude exceed the limits of the photographie 
aper. 
‘ ‘ In describing the magnetographs at Greenwich two scales were mentioned, 
one elastic, the other of paper, with which the times corresponding to the 
different points of the base-line were obtained, and the values of the ordinates 
of the curves calculated. These scales at Kew are metallic, and make part 
of an apparatus very simple and ingenious, which, being subject to a graduated 
moyement, is both easy and exact in operation. It is, however, not easily 
described without the assistance of a figure. 
‘For absolute determinations and secular changes there is a detached 
building of wood (copper-fastened) at a distance from the Observatory, where 
there are three wooden pillars solidly fixed in the ground, one for the instru- 
ments with which the coefficients of temperature and of induction of the 
magnetic bars are determined, and two for the inclinometer of Barrow and 
the unifilar of Gibson. These two instruments and a good chronometer 
constitute the necessary furniture of this building.” 
After a very careful and detailed description of the inclinometer and 
unifilar, Prof. de Souza proceeds, in his account of his first visit to Kew, as 
follows :— 
“In the verification-house, sixty yards from the observatory, Mr. Beckley 
was setting up for trial for the first time the registering electrometer of Pro- 
fessor Thomson of Glasgow. This new invention, which seems destined to 
supply a great desideratum in meteorology, would have been one of the objects 
of the greatest interest to me, if I could have seen it in action and have 
appreciated some of its results. Dispersed as were its different parts, I could 
not well make to myself a clear idea of the whole. The following is what I 
gathered from the explanations of Mr. Beckley. 
“Professor Thomson’s electrometer has for its object the photographic 
registration, by the system of Brooke, of variations in the difference 
between the electric tension of the atmosphere and of the earth. A 
semicircle of brass communicates with the earth; another semicircle of 
the same metal is insulated from the earth, and is in conmunication with 
the external air by means of the water of a reservoir, which is thrown into 
the air in a constant jet. From the top of the discontinuous circle formed 
by these semicircles, and in the direction of the space which they leave 
