1874.] Intensity of the Chemical Action of Total Daylight. 159 



exceed, a given tint. It becomes necessary therefore that on each occasion 

 when an observation is needed, the sensitive paper should be exposed 

 mechanically, not once, but for several known, but varying intervals of 

 time, quickly succeeding each other ; so that whatever may be the 

 intensity of the total daylight (supposed during these intervals to re- 

 main constant), some one at least of the several exposed papers will 

 possess the requisite shade. This is accomplished by a duplicate ar- 

 rangement of a clock and insolation-apparatus, by means of which disks of 

 the constant sensitive paper are exposed each hour for successive known 

 intervals of time, varying from two to thirty seconds. After an interval 

 of an hour, another set of disks are exposed for the same series of 

 intervals, and these series of insolations are repeated once every hour 

 during the day. The mechanical arrangements for effecting this with 

 accuracy are fully described in the paper. On unrolling, at the end of 

 the day, the strip of sensitive paper which has served for the exposures, 

 black disks showing where the paper has been stationary for the hour 

 are seen, and between each of these are found ten circles variously tinted, 

 from that, probably, scarcely visible, which was exposed for two seconds, to 

 that, perhaps too dark to read off, which was insolated for thirty seconds. 

 Amongst these, some one at least, will be found of such a shade as to 

 enable it to be read off by the monochromatic soda-flame, on a graduated 

 fixed strip, as described in former communications. 



A new method of calibrating the fixed strips of standard tints neces- 

 sary for these measurements is next described, and the question as to the 

 possibility of preparing constant sensitive paper in long strips instead of 

 in large sheets is next experimentally discussed, the result of the exa- 

 mination being that it is possible to prepare silvered paper in long narrow 

 strips such as are used in Morse's telegraph-apparatus, so that it shall 

 throughout its length preserve the standard sensitiveness. 



The time during which the disks of constant sensitive paper are ex- 

 posed is next ascertained for each instrument by a chronograph. 



During wet weather the insolator is covered by a semicircular glass 

 shade, and the value of the coefficients for refraction and absorption 

 due to this glass shade is determined. 



The latter portion of the communication contains the results of a series 

 of comparisons of the curves of daily chemical intensity obtained (1) with 

 the hand-ins olator, and (2) with the self-recording instrument. Com- 

 parisons of this nature were made during the months of May, June, 

 and July, 1873, by simultaneous hourly determinations in the neighbour- 

 hood of Manchester according to both methods. Of these observations, 

 six full days are selected, and the tables and curves accompanying the 

 communication show the close correspondence of both sets of observations. 

 The integrals of total chemical intensity for these days are also given, and 

 exhibit as close an agreement as, from the nature of the experiments, 

 can be expected. 



