672 



MR J. A. BROUN ON THE DIURNAL VARIATIONS OF THE 



well-founded wall. The daily variation of temperature within the closed room 

 was never more than a few tenths of a degree Fahrenheit. The value of a scale 

 division of Grubb's declinometer was 15"-35, and of Adie's, 16" nearly ; tenths 

 were easily estimated, and as the vibrations in Adie's instrument were generally 

 very small, if any, the mean of three readings was supposed accurate to one second 

 of arc. Both declinometers were observed hourly, and one was observed twice 

 at each hour, one minute before, and one minute after the other.* 



Diurnal Variations near the Magnetic Equator. 



Table I. — Hourly Variations of Magnetic Declination at Trevandrum, deduced from Observations 



during the twelve years. 1853 to 1864. 



Hour, 



























Trevan- 



drum Mean 



Time. 



Jan. 



Feb. 



Mar. 



April. 



May. 



June. 



July. 



Aug. 



Sep. 



Oct. 



Nov. 



Dec. 



H. M 



12 28 



1-34 



1-10 



0-47 



1-18 



1-64 



1-54 



1-42 



1-77 



1-66 



0-73 



1-44 



1-54 



13 28 



1-27 



1-07 



0-47 



1-27 



1-74 



1-67 



1-58 



1-89 



1-74 



0-72 



1-40 



1-47 



14 28 



1-15 



1-00 



0-38 



1-26 



1-78 



1-77 



1-66 



1-96 



1-77 



0-67 



1-27 



1-34 



15 28 



0-97 



0-86 



0-26 



1-15 



1-77 



1-80 



1-74 



2-02 



1-81 



0-57 



1-08 



1-15 



16 28 



0-73 



0-69 



0-13 



1-04 



1-78 



1-83 



1-82 



2T4 



1-85 



0-45 



0-85 



0-92 



17 28 



0-50 



0-51 



0-07 



1-13 



2-07 



2-10 



2-13 



2-54 



2-20 



041 



0-55 



0-70 



18 28 



0-20 



0-19 



0-00 



1-56 



2-85 



2-95 



2-97 



3-54 



3-19 



0-57 



0-08 



0-26 



19 28 



o-oo 



o-oo 



0-06 



1-67 



2-90 



3-06 



3-06 



3-64 



331 



0-59 



o-oo 



o-oo 



20 28 



0-64 



0-34 



0-27 



1-40 



2-33 



2-44 



2-35 



2 73 



2-51 



0-57 



0-43 



0-47 



21 28 



1-37 



0-86 



053 



1-11 



1-45 



1-56 



1-46 



1-62 



1-57 



0-43 



0-82 



0-93 



22 28 



1-46 



1-19 



0-73 



0-77 



0-65 



0-78 



0-73 



0-67 



0-79 



0-12 



0-95 



1-15 



23 28 



1-41 



1-11 



0-67 



0-26 



0T6 



0-20 



0-23 



0-12 



0-21 



o-oo 



1-10 



1-29 



28 



1-69 



1-25 



0-53 



o-oo 



0-00 



o-oo 



o-oo 



0-00 



o-oo 



0-28 



1-66 



1-78 



1 28 



2-06 



1-48 



0-51 



0-18 



0-42 



0-28 



0-18 



0-40 



0-44 



0-72 



2-14 



2-25 



2 28 



1-98 



1-47 



0-69 



0-47 



0-94 



0-67 



0'61 



0-98 



1-20 



1-07 



2-12 



2-33 



3 28 



1-86 



1-42 



0-79 



0-78 



1-35 



1-05 



1-11 



1-63 



1-85 



1-27 



193 



2-32 



4 28 



1-80 



1-37 



0-63 



1-01 



1-64 



1-30 



1-40 



2-08 



2-09 



1-12 



1-75 



2T8 



5 28 



1-57 



U24 



0-42 



1-02 



1-65 



1-33 



1-35 



2-05 



180 



0-73 



1-49 



1-84 



6 28 



1-54 



1-19 



0-44 



1-01 



1-45 



1-15 



1-21 



1-79 



1-70 



0-74 



1-57 



1-82 



7 28 



1-65 



1-26 



0-43 



0-77 



1-15 



0-89 



0-92 



1-50 



154 



0-71 



1-54 



1-81 



8 28 



1-57 



1-23 



0-40 



0-76 



1-11 



0-88 



0-84 



1-43 



1-50 



0-67 



1-45 



U71 



9 28 



1-49 



1-15 



0-37 



0-85 



1-24 



1-00 



0-95 



1-45 



1-47 



0-60 



1-38 



1-61 



10 28 



1-42 



1-15 



0-36 



0-98 



1-38 



1-17 



1-09 



1-52 



1-50 



0-65 



1-39 



154 



11 28 



1-39 



1-11 



0-46 



1-09 



1-54 



1-37 



1-26 



1-64 



1-60 



0-72 



1-46 



1-55 



The quantities in this table will be found projected, Plate XL., a considera- 

 tion of which has led to the following conclusions : — 



1st, The diurnal variation consists of one marked maximum and one marked 

 minimum of easterly declination in each month of the year, and of one or more 

 secondary maxima and minima. 



* The details connected with the description of instruments and the precautions taken to pre- 

 vent errors, will be found in the first volume of the Trevandrum Observations, which I hope will 

 soon be published. 



