350 REPORT — 1851. 



It is evident that, by this arrangement, B would descend through a space 

 equal to about six times the amount of the expansion of i" or 6'-, occasioned 

 by any given increment of temperature (a quantity equal to the difference of 

 expansion between zinc and mercury), provided that no expansion should 

 occur in 6^ 6'; but 6'< is made adjustable to a greater or less distance from 

 the fulcrum of i'% tor the purpose, not only of compensating the expansions 

 of b^ b'', but for endeavouring to correct other obvious little sources of error. 



Magnetographs. 



The Decimation Magnetograph, placed between the piers of the (trans- 

 ferred) transit instrument, and described in the Phil. Trans., Part I. 1847, 

 has undergone the following improvements : — 



The index (6') which was formerly used in producing the curve on paper, 

 has given place to a moveable shield, with its slit similar to 6* of fig. 1, 

 Plate v., Report for 1849. 



A diaphragm plate, similar to O, has been added, with provision for ad- 

 justing it, on the marble slab, in the direction of the length (of the slab). 



A fixed shield, like o\ has been attached to O, with the intervention of a 

 pair of sliders, moveable vertically and horizontally, by means of micrometer 

 screws, which allow it to be returned to its place if derangement in either of 

 these directions should occur; and the adjustments of O, on the slab, de- 

 termine its proper horizontal distance from b\ 



A socket has been fixed upon the lamp-support, which guides the lamp 

 into a constant position when exchanged at about sunrise for a plane mirror, 

 for which a similar kind of guide has been provided. 



This mirror is at present necessary for reflecting daylight into the camera 

 from a window (the locality not permitting direct daylight to enter it), and 

 consequently it is also necessary to keep the mouth at E much more opened 

 than the mouths of the other two magnetographs, in order to compensate the 

 loss of light by a longer exposure of the photographic surface in the slider 

 (H) to its influence ; this has (evidently) a bad eflfect upon the magnetic 

 curve, particularly on occasions of considerable and sudden magnetic dis- 

 turbances. 



The microscope (/«) has been made capable of being slid laterally, in 

 order that the image on the ground-glass (at e') may be viewed more di- 

 rectly ; for the principal part of what was before mistaken for other kind of 

 aberration arose in fact from the circumstance of viewing the image obliquely 

 through the microscope. 



The sliding-frame H has been provided with a scale, etched upon a piece of 

 ground-glass, to serve for verifying scale-coefficients, &c. The scale is divided 

 to fiftieths of an inch, corresponding to the divisions of the T square used 

 with the scale-board (fig. 2, Plate IV., Report for 1819). Also an additional 

 pair of rollers like 6^ fig. 2, has been attached to the left side of H ; and an 

 additional ring has been fixed on the bottom, in order that the frame may be 

 employed in an inverted position in the slide-case (F), and that thus the 

 trouble of either preparing two plates per diem, or the necessity and risk of 

 withdrawing from and replacing the one plate in H (at 12 p.m.) might be 

 avoided*. . 



These and some other little ameliorations in this instrument were made 



* It was not originally intended that the plate should be inverted {vide fig. 2, Plate XXI.) 

 for the purpose of procuring the two new lines and the two curves on it. The effect of the 

 new arrangement has been, in a few instances of magnetic disturbance, to contract the field 

 on the plate for the range of the image of the slit so much as to exclude a part of the curve. 



