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



vegetable and mineral salts, &c., fluorescent and non-fluorescent, 

 variously polarizing and non-polarizing, &c. Such observations 

 might also probably give interesting results as connected with 

 substances of one composition, but differing allotropically. Using 

 various liquids in this way, we should, however, of course require 

 to make allowance for the effects of differing specific gravities, 

 either in reading or in translating the observations, or otherwise*. 



As to the precise form of this apparatus and mode of register- 

 ing, whether photographic or other, the principles once explained, 

 it would be occupying space to perhaps no great purpose to 

 enter into pretty obvious details and variations. If the sensitive- 

 liquid reservoir, if below the water (or the float attached to it, if 

 above), has attached to it a descending rod which fits into, and is 

 held perpendicular by, a ring or rings, and if another rod attached 

 to the float or sensitive-liquid vessel above the water is similarly 

 kept in its upright position by similar rings, it is obvious that 

 the upper rod affords very convenient means of registry, either 

 by having the scale on itself, with a marker pointing to it attached 

 to the side of the outer vessel or other fixed object, or by making 

 it cany the marker (in this case a shifting one) and making the 

 scale attached to the outer vessel (or otherwise) fixed and steady. 

 As to the adaptation of photographic self-registry on sheets or 

 slips of sensitive paper, I need hardly say that the upper rod is 

 capable of performing here exactly the same part which the 

 column of mercury, oil, or other register-liquid performs in the 

 earlier-described apparatus. 



With regard to all the apparatus described, it would be in 

 most cases, especially where used for protracted observation, 

 advisable to have them kept in one angle, as regards the sun, by 

 some sort of heliostat. It might, however, be found practicable 

 in consecutive, independent, and even perhaps in continuous 

 observations, to avoid the necessity of this by having the sen- 

 sitive vessel, or that part of it which is exposed to the rays, of a 

 globular form, so as to make their incidence similar at all hours. 

 In this case, however, it must be observed that the part exposed 

 to the rays must be filled with liquid ; otherwise, however simi- 

 larly they might fall on the surface of the glass, we should still 

 have them impinging at varying angles on the plane upper sur- 

 face of the contained sensitive liquid. Where the weight and 

 floatation-system is employed, this plan is not quite so conve- 



* We might avoid this, or the necessity for other scales, simply by adding 

 to, or deducting from, the weight of the sensitive fluid and its reservoir till 

 it floats or sinks to the same depth in the liquid to be experimented on as 

 it does in water. This will be conveniently managed by having a little cup 

 attachable to some part of the floating apparatus, into which we may put 

 more or less sand, removing or adding it till the apparatus floats at the 

 proper line marked on or by it. 



