MAGNETIC REGISTRATION. 183 



confess is an improvement, that is, instead of having a concave mirror 

 attached to the magnet, there is a plane glass mirror, and the light 

 is collected into a focus by means of a fixed combination of lenses. 

 The advantage is that the mirror is much lighter, the magnet is not so 

 much loaded, and you then get a direct image in which the axis of the 

 pencil of light falls centrally on the paper ; whereas by the system I have 

 adopted you get to a certain extent an oblique reflection, and the image 

 so obtained is not so distinct as that obtained by direct reflection 

 or refraction. 



There is a little additional apparatus here for the purpose of 

 testing the scale coefficient, or value of the displacement of the in- 

 dication on the paper, by adding a minute weight of '01 of a grain in 

 this scale. You will observe how much that displaces the bar, and 

 thence obtain the value of the ordinates of the curve. I have only 

 further to say that the magnets in some cases are flat bars ; but as 

 set up by myself at the Paris Observatory, they are of this form a 

 hollow magnet, with a lens at one end and a collimater scale at the 

 other, so that the photographic records may always be checked by 

 observations with the telescope of the magnet itself. To each in- 

 strument a small plane mirror is also attached, so that the indicated 

 amounts of variation may be checked by a fixed telescope and a scale. 



The CHAIRMAN : I am sure you will all agree with me that we are 

 much obliged to Mr. Brooke for his very able description of the 

 photographic recording instruments, to which he has so largely con- 

 tributed. Mr. Ronalds, formerly director at Kew, he has already 

 alluded to, but since Mr. Ronalds' time the directors of Kew have not 

 been idle ; the late Mr. Welch contrived the above-mentioned modifi- 

 cation of this very beautiful instrument of Mr. Brooke's, and a complete 

 set will be shortly erected in the grounds of the observatory. The Kew 

 instrument is now being worked under the direction of Mr. Whitwell ; 

 and beside that there are instruments at St. Petersburg, Lisbon, Coimbra, 

 Florence, Toronto, and in many other places. It is extremely important 

 that simultaneous and continuous records of magnetic changes should 

 take place at a great number of points on the earth's surface. We have 

 the curves recorded at Lisbon and at Kew, and the disturbances are 

 found to take place at the same time, and prove that magnetic changes 

 are cosmical and that they are synchronous that is to say, the instru- 



