1869.] 



of the Solar Prominences. 



65 



sights themselves are so beautiful and interesting that no other incentive 

 is needed. This morning I showed a magnificent prominence up- 



wards of 3' in height ; and she testifies that my sketches do not do them 

 justice. 



The instrument I am now using is the Royal Society's spectroscope as 

 fitted up for the eclipse ; but I have increased the dispersion nearly three- 

 fold by inserting four compound prisms (extracted from the hand-spectro- 

 scopes). These amount to 7 inches of glass and sixteen surfaces ; so you 

 may imagine that there is some loss of light and definition. I have also 

 had to shorten the focal distance (and therefore diminish the magnifying- 

 power) by interposing a hand-telescope's object-glass — an additional ob- 

 struction and complication. I lost a great deal of fine weather (of which I 

 get very little now) while trying to perfect this arrangement. I can still 

 further increase the dispersion (without much loss of definition for mono- 

 chromatic light) by turning the main prism, and so departing from the po- 

 sition of minimum deviation. But this is a resource which I keep to go 

 on with when I tire of the advantage I have gained already. 



The long train of compound prisms (as at present arranged) unfortunately 

 bars me from the violet end of the spectrum. This is unfortunate, as it 

 would be in the highest degree interesting to compare the a and y images. 

 Some day I shall get impatient, pull the whole affair to pieces, and arrange 

 afresh with this object. As it is, I have to be very chary of quitting 

 beaten ground, as we boast of no instrument-makers here ! 



I wish I had time to write fully and connectedly on the subject. It is 

 only necessary to put people on the track. It is one easily followed, and 

 will amply repay any expenditure in arranging prisms to get a maximum 

 dispersion, for there is any amount of light. 



XXVII. " Some Experiments with the Great Induction Coil at the 

 Royal Polytechnic." By John Henry Pepper, F.C.S., Assoc. 

 Inst. C.E. Communicated by J. P. Gassiot, Esq. Received 

 June 12, 1869. 



The length of the coil from end to end is 9 feet 10 inches, and the dia- 

 meter 2 feet ; the whole is cased in ebonite ; it stands on two strong 

 pillars covered with ebonite, the feet of the pillars being 22 inches in 

 diameter. The ebonite tubes &c. are the largest ever constructed at Silver- 

 Town Works. 



The total weight of the great coil is 15 cwt., that of the ebonite alone 

 being 477 pounds. 



I am indebted to Mr. Apps for the following details. The primary wire 

 is made of copper of the highest conductivity, and weighs 145 lbs. ; the 

 diameter of this wire is *0925 of an inch, and the length 3770 yards. The 

 number of revolutions of the primary wire round the core of soft iron is 

 6000, its arrangement being 3, 6, and 12 strands. 



VOL. XVI 1 1. F 



