58 Mr. C. J. Burnett on several Forms of Actinometer. 



with still intervals between, might be preferable ; and this latter 

 plan would give us, in different registers, the average of each 

 second in the minute, each minute in the hour, hour in the day, 

 day in the week or month, and so on, — the average or mean, as 

 compared with other days on the same week's or month's regis- 

 try-slip, giving us at once the true value of the depth of tint in 

 the whole and every part of that day's own separate registry, 

 and so giving it a real utility which, taken by itself and without 

 being thus brought to the test, it could never have had ; and (on 

 the same principle) so on with week-, month-, and, it may be, 

 year-register papers. It must be allowed, with all their recom- 

 mendations, that it would be desirable to get papers which would 

 keep better than the ordinary silver ones, and which would 

 suffer less in fixing. 



As long ago as 1857* I called attention to the probable 

 value, both for actinometry and for all other self-acting scien- 

 tific registry by time, of papers prepared with the salts of 

 ferric and uranic oxides. I had then, as I remarked, found 

 them to retain their properties unimpaired for between one and 

 two years, and I have since found them good when four years 

 old. This ought to save a good deal of labour, by enabling us 

 to prepare at once and keep ready a large stock of them. I am 

 not, however, prepared to state what their relative advantages as 

 compared with silver papers are, as regards keeping after 

 solarization and before development. The development is, I 

 think, more uniform in its results than that of the silver papers, 

 being performed with nitrate of silver alone. The time of keep- 

 ing without a material change after solarization (or the amount 

 of change) would require to be more minutely examined before I 

 could speak veiy positively as to the advantage of their applica- 

 tion to the papyro-actinometer ; but for the paper-registry of 

 the gas-evolving actinometers, whether gauged by floating or 

 by fluid columns, as well as for self-registry of thermometers, 

 barometers, anemometers, &c, there can be, I think, little room 

 to doubt that these papers will be found particularly convenient. 



It is perhaps hardly necessary to remark that one clock, or other 

 moving power, may of course be made to move all the different 

 time-registries we have recommended, each one at its own proper 

 rate. 



A very convenient arrangement for a set of paper-slips would 

 be to have one a continuously moving slip ; the second one 

 moving by sudden jerks, taking place every minute or every five 

 minutes, and so giving the average of, say each minute or five 

 minutes ; the third a similarly jerked slip, giving the average 

 or mean for each hour ; and the fourth (say a week- or month- 

 * Even in 1855 I alluded to the possibility of their availability. 



