Mr. R. Phillips on the Magnetism of Steam. 503 



and fastened to the triangular bar by means of cork and thread, 

 and a hole about an inch across was cut in the card cover, 

 the centre of the hole being over the point of the uppermost 

 needle ; the arm for carrying the optical tube was now brought 

 over the hole, and then the optical tube, fitted with an object- 

 glass, was screwed into its place ; the arm came down nearly 

 close upon the card cover, and the tube of the object-glass 

 passed through the cover and stood over the point of the upper 

 needle. The fine adjustment was used to focus the instru- 

 ment; and to bring the point of the needle to its right position 

 under the object-glass, I employed the motion of the arm 

 which carried the optical tube, and also that imparted to 

 the whole instrument by laying hold on the extremity of one 

 of the prongs of the tripod stand and making it describe a 

 small portion of a circle on the table. The point of the needle 

 was viewed by means of a pencil of light thrown from the 

 mirror through the bottom of the beaker. The magnifying 

 power used was about 450 diameters. 



4. A micrometer eye-piece was employed, the scale of which 

 formed an angle of about 25° with the edge of the needle to 

 be observed. The optical power of this micrometer corre- 

 sponded to the third, or deepest eye-piece, with which micro- 

 scopes are generally furnished ; because that for such experi- 

 ments a shallow object-glass is to be preferred. 



5. This galvanoscope was sheltered by a rectangular zinc 

 plate, '1 inch thick, bent somewhat into the shape of the smaller 

 segment of the convex surface of a right cylinder, made by a 

 plane parallel to, and at a considerable distance from, its axis. 

 The length of a line drawn perpendicularly from one straight 

 side to the other was 10 inches, and the maximum length of a 

 perpendicular drawn to this line and extending to the nearest 

 point of the zinc was 2*8 inches; in consequence of this curve, 

 the zinc plate could easily be made to stand on end, in which 

 position it was 18*5 inches high. This shield was employed 

 throughout all the following experiments on steam. 



6. In order to obtain accurate results, I found it necessary 

 to avoid moving any mass of iron near the galvanoscope, and 

 also to keep moderately still, although the hands and arms 

 could be freely used without affecting the galvanoscope. 

 During these experiments I sometimes observed a very singular 

 effect produced on the magnetic needles: when the steam was 

 turned on, the needle would begin to move across the field of 

 view with a peculiar slow motion ; and when the steam was 

 shut off, it as slowly or more slowly returned to its former 

 position, one of such vibrations occupying half a minute or 

 more. This I at length found was produced by the steam 



