880 REPORT — 1851. 



several observations it was found that the ordinate of j^th of an inch 

 =2'"192, agreeing very closely with the value obtained by the other method. 

 The optical value for joth of an inch=2'*19. 



The value of the angle of torsion of the suspending wires was determined 

 by a method quite similar to that described in the ' Report of the Committee 

 of the Royal Society.' The arm carrying the moveable shield has a motion 

 in azimuth similar to that of the collimator in Dr. Lloyd's form of the bifilar 

 magnetometer ; the position of the image of the slit, with reference to the 

 divisions of the ground-glass scale, serving all the purposes of the telescope 

 and scale. The angle of torsion in the present adjustment was found to be 

 e^" 45'. By the usual formula, we find the value, in parts of the whole hori- 

 zontal force, of an ordinate of ^^th of an inch=0'000300. 



The effect of temperature upon the magnetic moment of the bar was exa- 

 mined in December 1850 by the usual method of deflections. The results 

 were — At temperature 55°'3, the effect of one degree =0'0003 12 ; and at 

 temperature 76°'7, the effect of one degree=0*00034'4'. 



The temperature of the magnet is obtained by a thermometer whose bulb 

 is within the box. Observations of the thermometer were taken generally 

 every three hours, rather more frequently during the day, and not so often 

 at night. They were taken usually by myself during the day and until mid- 

 night ; whilst Mr. Nicklin, whom I had instructed so as to observe the ther- 

 mometer with accuracy, took them at early morning or during my occasional 

 absence. 



The effect of the illuminating lamp in heating the air within the magnet 

 box has been found to be rather considerable. At the commencement of 

 the series, when no precautions had been adopted to prevent this effect, the 

 thermometer showed that the air was heated about 4 or 5 degrees above 

 what it would have been had the lamp been away ; wooden screens were 

 afterwards interposed for the purpose of preventing radiation from the lamp, 

 but the effect was still about 2 or 2^ degrees. 



The effect of the new copper damper in checking mechanical oscillation 

 was found to be very striking, a large arc of vibration being reduced to 

 nothing in four or five swings. 



The length of time during which the light of the registering image acts 

 upon the plate is about 1| minute. 



Vertical-Force Magnetograph. — The agate planes, upon which the knife- 

 edges of the magnet rest, were made horizontal by means of a level supplied 

 for the purpose by Mr. Barrow, and the instrument brought into approxi- 

 mate adjustment. The distance from the centre of motion (the knife-edge) 

 to that portion of the slit in the moveable shield which produces the image 

 on the plate, was thus obtained : — a brass rod carrying a shield, with a slit 

 similar to that attached to the magnet, was made to rest, in an upright posi- 

 tion, upon the agate planes, by means of a cross piece having a flat base 

 which occupied the same position as the knife-edge of the magnet when ia 

 adjustment. The image of a portion of the slit is formed upon the focua 

 glass : a fine point was then moved slowly along the slit by one person until 

 another observed the image of the slit bisected by that of the point. A mark 

 was there made on the shield. The distance of this point from the base is 

 equal to the distance from the centre of motion to the effective portion of 

 the slit in the moveable shield, and may readily be determined by a scale and 

 square. It was found to be ir93 inches. The magnifying power of the 

 lens was found by the method already described to be 3*78. From these 

 quantities we have the value in arc corresponding to an ordinate of j^th of 

 au inch =1 '•525. 



