ON THE KEW OBSERVATORY. 127 



Mr. CoUen's photographic impression of a one-hour plate, which was fixed 

 upon the minute arbour of the clock, is also preserved*. 



A Leyden jar, of about -tO inches coating, has been sometimes used for 

 receiving the charges from the rod, and the number of discharges up to the 

 maximum tension of the rod in a given time has furnished a better estimate 

 (in very high tensions and quantities) of frequency, than we at present other- 

 wise arrive at perhaps. 



A PAIR OF BELLS has been sometimes applied to the conductor in the usual 

 way, but they are too small to give us due notice below of high charges. 



An Argand lamp is burned at about 3 feet from the conductor in the 

 evening, for lighting the electrometers, &c., and a little chimney placed above 

 it, and opening outside the dome to prevent hot air and vapour from ap- 

 proaching the conductor, or anything connected therewith. 



A SMALL Joyce's stove, containing a little burning charcoal at night, is 

 generally suspended in the dome for keeping eveiything dry. 



Great care is requisite, and diligently observed by Mr. Gallowaj', to guard 

 as much as possible the whole apparatus from dust. He uses occasionally 

 soft camels'-hair brushes. 



I believe that every article which has been used, more or less constantly, 

 for the electric observations of the tables, has now been shortly described. 



I placed A condenser in the room, but we have not used it. I think that 

 Volta's objections to the employment of such instruments in comparative ex- 

 periments are founded in sound reason and experience. 



Barometers. 



T/ie mountain barometer, lent by Colonel Sabine until we can afford the 

 expense of a standard instrument, has been used since the commencement of 

 the observations here ; it is by Newman ; the graduated scale is divided to 

 0"05 of an inch ; the vernier subdivides the scale divisions to 0*05, and is 

 moved bj' a slow screw. 



The particulars given, for corrections, are as follows : — 



Capacity 1*5.5 



Neutral point SQ'TG^ 



Capillary action +0*043 



Temperature 55° 



It is freely suspended by a ring in the mural quadrant-room B (fig. 1), 

 near the north window. It has been compared with the barometer of the 

 Royal Society, and the comparison is recorded there. 



The observations are set down without corrections of any kind. 

 A centigrade barometer hangs freely in the dome, but we use it for casual 

 observations merely, and seldom. 



Thermometers. 



The thermometer which we call our standard, by Newman, is mercurial, 

 and divided to 0'5 ; it has not been compared with others. It is fixed at the 

 outside of the north window of the apartment B (fig. 1). 



The maximum thermometer is mercurial ; it is made by Newman, is 

 divided to 0"1, and the index is of blue steel. It is placed outside the north 

 window of the room B (fig. 1), near the standard thermometer. 



The minimum spirit thermometer, by Newman, is divided to 0"1, and the 

 index is of black glass ; its position is nearly the same as that of the maximum 

 thermometer, i. e. on the opposite side of the same window. 



* This kind of graphic exhihition is perhaps more pleasing but less useful than other modes 

 of registration -which we hope to accomplish. The tsedium and difficulties of bringing the 

 resinous coating to a uniformly fit state for receiving the electrical dra^ving are not incon- 

 siderable. 



