ON THE KEW OBSERVATORY. 337 



The three Night-Registering Electrometers, described at p. 139 of the Re- 

 port for 1844, are effective, but little employed. They were very useful 

 formerly, but will soon give place to the Photo-Electrograph. 



The Gold-leaf Electroscope itself remains nearly as described at p. 125 of 

 the same volume. The little additional apparatus for preserving its insulating 

 power, afterwards alluded to, and now described more particularly, has been 

 found very convenient and effective. 



A (Plate XVIII. fig. 2) is a thick plate of well-ground and polished glass. 



B, a kind of annular tin trough, coated with sealing-wax varnish, and con- 

 taining chloride of calcium. 



C, the electroscope. 



On the central part of A, not occupied by B, stands C ; which, together 

 with B, is covered by a glass receiver, fitting air-tight upon A when C is not 

 in use. 



By this means the electroscope may be preserved in a dry and clean state, 

 and quite ready for use at any moment ; and it is evident that a similar 

 drying arrangement may be adopted in respect of a Volta, or any other de- 

 tached electrometer or electrical instrument, &c. 



The pair of Portable f^olta- Electrometers, occasionally used on the leaden 

 roof of the building for experiments on induction, absorption, &c. of atmo- 

 spheric electricity, are in the original state alluded to at p. 140 of the Report 

 for 1844; they were not particularly described there, because, with the ex- 

 ception of a few additions and alterations, they are similar to the instruments 

 used in the dome ; but several eminent meteorologists having thought that 

 these instruments would, if used with proper precautions, afford better ap- 

 proximative results in observations, on moimtains, &c., tiian the portable 

 instruments which have been usually employed, the following short but com- 

 plete account of them may possibly be found convenient. 



A (Plate XVIII. fig. 1) is the lower part of one of them. 



a', a little hollow pedestal, the side, base, and upper surface of which are 

 formed out of one brass casting. It is about S inches high and 2 inches 

 square. 



a^, a cupola of cast brass screwed firmly into its upper surface. 



a^, a lamina of thin plate-glass polished and attached, by a frame of brass 

 and screws, to a* : a lamina of ground glass is fixed in like manner to the back 

 of a'. 



a*, a tube of thick glass well-coated with shell-lac, applied by heating the 

 glass until it is capable of melting the lac, but not of carbonizing it. It 

 passes through and is firmly cemented to a cover which is screwed into a'^. 

 Its lower end projects about an inch below the cover, and on the upper end 

 is cemented a brass cap and screw. A wire, forming a continuation of that 

 screw, passes through the bore of a*, in which it is securely fixed ; this wire 

 terminates below in a flattened part, which has two minute perforations at the 

 distance of half a Paris line from each other. 



a% a pair of Volta's straw penduletti, two Paris inches long. In their 

 upper ends are fixed hooks of fine copper wire, which pass through the per- 

 forations and suspend the straws freely : their lengths and diameters are in 

 strict conformity with Volta's prescription for his standard instrument 

 (No. 1). 



a^, an ivory scale fixed in front of a^ ; its upper edge is an arc whose 

 radius is equal to the lengths of a=, and it is graduated in half Paris lines ; 



1851. . z 



