^^'^l^Reghlcring the Thermometer and Barometer. 275 



shadow was not perfectly continuous, but, as can be perceived, 

 a series of white spaces were interposed between each of the 

 dark portions. The translucency of the coloured spirit also 

 permitted some light to act upon the lower part of the sheet, 

 making faint prolongations of the dark spaces, 

 or In my first experiment the temperature was registered in 

 this way for only five hours ; in my second, the result of which 

 is exhibited in the diagram, the operation was carried on for 

 nine hours, or from ten in the morning until seven in the af- 

 ternoon. During the course of the last experiment, the sky 

 was occasionally much obscured so as to lessen the intensity 

 of the light, and botiieKpei;in;hent&w>eire;' performed during the 

 'pi'esent monlh.ijjqqn :jiU no Hni Ion bib nii>, . 

 h When results so satisfactory have been obtained with such 

 itnperfect apparatus as that which I have used, it is super- 

 fluous to anticipate what would take place under more favour- 

 able circumstances. The application of photography to re- 

 gister the height of the mercury in the barometer ought to be 

 still more practicable than in the thermometer, as in the 

 former case the shadow thrown on the paper would be so 

 much broader than could ever be possible in the latter. I 

 trust that I shall soon be able to present such facts on this 

 portion of the subject as will effectually clear up all doubts. 

 The experiments which I have made on the application of the 

 Daguerreotype and calotype, although yet incomplete, have 

 been satisfactory. From the researches of Prof. Draper and 

 others on the action of artificial light upon Daguerreotype 

 plates, I have been led to hope that temperatures and baro- 

 metrical oscillations can be registered continuously at night 

 as well as by day. In using a lamp to produce a shadow of 

 the fluid column in the instrument, it may at first appear that 

 errors would be produced, from the circumstance that the 

 shadow would not always have the same proportion to the 

 column. But that such errors would be inconsiderable ap- 

 pears from the easily deduced formula 



! l + m ' 



where in the case of a thermometer @ represents the number 

 of degrees in the true temperature, 5 the number recorded on 

 the plate, H the height of the bulb of the instrument above 

 an horizontal plane, h the height of the flame of the lamp 

 above the same plane, I the perpendicular distance from the 

 flame of the lamp to a plane passing through the mercurial 

 column parallel to the plate, and vi the distance from the mer- 

 curial column to the plate. A similar formula would give the 



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