60 MA<; N E TIC S U 11 V E V F J' E N X S Y L V A N I A . 



Relative Intensities by tin- Lloyd Needles. 

 Tempt rature correction. — For the old London weights. 



No special observations to determine the temperature coefficient have boon made, 

 we may deduce it, however, from the following combination of observations at the 

 Girard College Station, Philadelphia: — 



For Floyd Needle No. 1. 



July21,1840, t=76°.5 «=_l°09'.!i (weight in third hole). 



on. 28, 1840, 52.0 + 1 28.9 



it = 24.5 A9=- 2 38.8 

 hence A o = — 6.5 A t 



lu Vol. II of the Magnetic and Meteorological Observations at Girard College 

 (p. 1537), we find the additional observation: — 



July 20, 1843, t = 73°.2 9= + 0°42'.7 (position of weight not stated). 



For Floyd Needle No. 3. 



July 21, 1840, t = 77°.0 fl = + l°09'.l 



Oct. 28, 1840, 50.0 + 2 37.2 



At = 27.0 A9 = — 1 28.1 

 hence a = — 3.3At 



In Vol. II of the Magnetic and Meteorological Observations at Girard College 

 (p. 1537), Ave find the additional observation : — 



July 20, 1843, t =75°.8 8 = + 4° 4S'.0 (position of weight not stated). 



The mean of both needles gives = 0'$ 1-4'. 9 (t—f) I 



The mean temperature would have to be taken as the standard temperature to 

 which all observations for relative horizontal intensity are to be referred. 



\ . B. For the old weights the sign of the angle has significance. 



These weights wore also used in Europe. 



We may safely assume that within the interval of the survey the needles have 

 not perceptibly lost in their magnetism. The total intensity is also very nearly 

 constant for any one place. 



Temperature Correction. — For the new or pin weights. 

 Lloyd Needle No. 1. 



Oct. 28,1840, t = 52°.0 = 17°15'.3 (pin in third hole). 



\pril 26, 1841, 68.0 18 06.7 



July 20, 1841, 92.5 17 54.5 



Lloyd Needle No. 3. 



Oct. 28,1840, 1 = 50°.0 ?=19°23'.0 (pininihinlh.de) 



April 26, 1841, 68 13 55.9 



.July 20, 1841, 87.6 19 31.4 



From these observations it would appear that the incidental errors of observation, 

 or other accidental causes, exercise a greater influence on the resulting angle than 

 the change due to changes of temperature within the above range. 



