364 REPORT— J 851. 



In June a little progress was made in the construction of a new Declination 

 magnetograph, or rather in new mechanism, to be substituted for some parts 

 of the old declination instrument. 



From April 1 to this time, i. e. during the course of the experimental trials 

 of the three magnetographs, the Daguerreotype plates prepared by Mr. 

 Nicklin, our photographist, were put into the instruments daily, with the few 

 exceptions stated, at meridian as nearly as possible, the exact time (by Dent's 

 chronometer, with proper corrections) being known for each. They were 

 inverted at 12 p.m. (as nearly as possible). 



The course of experimental trials for six months was undertaken in accord- 

 ance with a wifh expressed by the Kew Committee. I should have been 

 better pleased if I could have previously substituted a declination magneto- 

 graph, similar in general construction to the horizontal-force magnetograph, 

 instead of the original instrument set up before many of the improvements since 

 adopted had been made, and if I could have attempted other improvements 

 on all the magnetographs (as that of substituting pointed for rectangular 

 magnets, in order to diminish the time of vibration, &c.). The Royal Society 

 granted me, in the kindest manner, a sum not exceeding £100 to be applied 

 to the costs attending the trials, of which sum about one-third has been ex- 

 pended (and an exact account with vouchers maintained). 



Thermometers . 



On September 21 and 23, comparisons of Newman's standard thermometer 

 were made with various thermometers ; their index corrections are recorded 

 at pp. 342 and 343. 



On February 22, the dividing machine by M. Perreaux, together with 

 the adaptations and other thermometric apparatus of M. Regnault's (alluded 

 to at p. 346, ante), arrived, and soon afterwards Captain Lefroygave some ex- 

 planations concerning the former, and made satisfactory trials of their appli- 

 cability to the accurate calibration and division of thermometer tubes. In 

 these and subsequent trials a diamond point was sometimes used, and when 

 the etching process, by means of hydrofluoric acid, was preferred, some diffi- 

 culty occurred as to the choice of a proper "ground." Oil of Spike (lavender) 

 was found to answer the purpose tolerably well. An incomplete attempt 

 was made to etch plates of glass by Fluorine gas. Some tubing, entirely of 

 vulcanized India-rubber, was advantageously substituted for the original 

 tubing compounded of this material and glass in the operation of moving 

 the calibring mercury in the thermometer tubes*. 



On February 23, the maximum Rutherford thermometer (F) was found 

 to have been destroyed. 



On April 3, the Sub-committee of the Kew Observatory, appointed to 

 direct the construction and verification of standard thermometers, &c., 

 made experiments for determining the freezing- and boiling-points of a 

 standard thermometer of M. Regnault, No. 229, or Woolwich standard, 

 which Colonel Sabine brought (with another by Ronchetti) ; also of 

 the standard Regnault, No. 231 (or Kew standard). The freezing-points 

 of the above Ronclietti, Newman's standard (C), the immersed ther- 

 mometer in the improved Regnault hygrometer, and the immersed thermo- 

 meter of the new Daniel hygrometer, were also determined. The apparatus 

 used was a part of that referred to at p. S46, and the results obtained are 

 already stated, excepting that the boiling-point of the Regnault No. 229, or 



* ^ further improvement for effecting this object is contemplated. 



