360 REPORT— 1851. 



raised about 4° by the heat of the lamp. It was found that the time of passage 

 of a given point over the aperture of the mouth was about 1^ minute. 



On April 1 at m. it was put in motion for the due prosecution of the pro- 

 posed experimental trials. 



About the 2nd of May a cessation of its activity occurred for two days, 

 spent in minor improvements, as that of substituting the larger thermometer 

 by Gary, for that previously used, &c. 



From April 1 to this time, i. e. July 1, the thermometer has been read at 

 intervals of about three liours. 



The Daguerreotype curves, &c. produced daily, with the above-mentioned 

 exception, from the commencement of the series of trials to the present time 

 by means of this instrument, have almost constantly afforded means of mea- 

 suring ordinates (in the manner described at p. 9 of the Report for 1849) 

 to -^lodth of an inch, and generally of describing ckar gelatine tracings (in 

 the manner alluded to at p. 185 of the Report for 1850). 



Vertical- Foice Magnetograph. 



In September and November 1850, drawings and instructions were given 

 to Mr. Ross, Mr. Barrow, and others, for constructing principal parts of 

 this new instrument ; preparations were made for its reception ; and work on 

 it proceeded here. 



On December 14, its magnet, &c. having arrived, experiments for deter- 

 mining its temperature coefficient were made at the same time as, and in 

 like manner to those on the horizontal-force magnetograph ; when nine ob- 

 servations, at the mean temperature of 50'**4, gave O''*000283, and II obser- 

 vations, at 71°-5, gave 0°'000319. 



On the 21st Mr. Ross's part arrived. 



On February 8, the apparatus described at p. 352 having been completed, 

 observations were made to determine the magnifying power of the lenses, and 

 to examine the effect of distortion by its means. The results from two series 

 of observations were, that the magnifying power was 3'81 times, and that " the 

 aberration was very small, so small indeed as to render it uncertain whether 

 the image, in the middle of the field, was greater or less than at the ends; 

 the whole amount of apparent difference being quite within the probable 

 error of observation." In this case the ground glass plate, at the mouth (E), 

 was divided to ^th of an inch*. 



On February 10, it was ascertained that no sensible difference in the Da- 

 guerreotype impression was produced by enlarging the aperture at the object- 

 end (C) of the camera. 



On February 24, measurements were made to ascertain the radius, 

 viz. 11*9 inch, from the knife-edge to that part of the slit in the moveable 

 shield (o') which corresponds with the moveable image in the conjugate 

 focus at the mouth-piece (E), by means of the instrument described at p. 352, 

 ante. 



On the 25th this magnetograph was mounted upon its corbels (in the qua- 

 drant room) and partially adjusted for work. 



On March 3, the transit of a given point of the Daguerreotype plate over 

 the mouth (at E) was made to occupy 1^ minute. 



On the 10th it was ready for further trials, and on this day a good curve 

 was produced; but soon afterwards (on the 19th) we found that the curves 



* The spherical aberration of the lenses was not examined. 



