6 Mr. R. Potter on a New and Simple Heliostat. 



By means of these data we obtain for the total magnetic 

 intensity at the beginning of the present year 1832, — 



Paris. Brussels. Gottingen. Berlin. Stockholm. 



1-0000 10205 1*0010 0-9982 1-0340 



The result for Berlin differs greatly from that which might 

 have been expected from the general decrease of the intensity 

 with the latitude. I have no reason, however, to suppose 

 that there is any error in the observations. The dip was de- 

 termined by four trials, as well by myself alone, as by M. Riess 

 and me; and the differences between the results were ex- 

 tremely small. 



Stockholm, Sept. 20th, 1832. 



III. On a New and Simple Heliostat. By R. Potter, Esq., Jim* 

 [With Figures : Plate I.] 



HAVING a short time ago determined upon commencing 

 a series of optical experiments, which will require the 

 use of an instrument having the effect of a heliostat, my first 

 step was to seek to make myself acquainted with the principle 

 of that of Dr. S'Gravesande. This is the only instrument of 

 the kind of which I have yet met with any account ; and, by 

 accident, the work which I consulted for a description of it 

 having the plates bound in a separate volume, I could only 

 at that time have access to the description without the plate. 



Those who know Dr. S'Gravesande's instrument will not be 

 surprised that I should soon be embarrassed in consulting a de- 

 scription intended only to be intelligible with the assistance of 

 the figures. I had not, however, got through the account, when 

 a thought struck me that the simplest plan of a heliostat must 

 be on the equatorial principle : another moment's reflection 

 convinced me that a very simple, yet effective construction 

 might be adopted upon that method. 



I have since made a heliostat upon this plan ; and having 

 proved its efficacy, I now proceed to give a description of it. 

 The instrument I have executed is similar to figures 2. and 3, 

 of which fig. 2. represents the side view, and fig. 3. the hori- 

 zontal one. Before, however, we go to the description of the 

 instrument itself, we will examine the principle of its con- 

 struction. 



Let eo 9 fig. 1. be a spindle which can be made to revolve, 

 and which is set parallel to the earth's axis. Let a b be a 

 mirror attached to the end of the spindle. Then the appa- 



* Communicated by the Author. 



