The Astronomer Royal on the Strains in the Interior of Beams. 151 



actual applications it is necessary to determine some remaining 

 constants by considerations peculiar to each case. Now there is one 

 modification of the strains whose value can be ascertained by ordinary 

 mechanics, namely, the horizontal part of compressive force in the 

 part of the beam above the neutral line, and the horizontal part of 

 tension force in the part of the beam below the neutral line. (These 

 words apply to a beam supported at both ends ; in the case of a beam 

 projecting from a wall, the words "compression" and "tension" 

 must be reversed.) By determining the corresponding expression 

 on the theory of this memoir, and comparing the two, the remaining 

 constants and the form of the function are completely determined. 

 From its partial differential coefficients are found the three functions 

 depending on the forces acting at any one point (as already men- 

 tioned), and from these three functions are found the magnitudes of 

 the two principal forces of compression or tension, and the angle 

 which one of them makes with y, in a form admitting of numerical 

 calculation. 



The author then applies the theory to six cases, namely, (1) a beam 

 projecting from a wall ; (2) a beam supported at its ends ; (3) a beam 

 supported at its ends and carrying a load on its center; (4) a beam 

 supported at its ends and carrying an excentric load ; (5) a beam 

 strained at both ends by the connexion of other beams, in the manner 

 of the tubes of the Britannia Bridge ; (6) a beam strained at one end 

 only. Cases (3) and (4) require the use of discontinuous functions. 

 Tables are given, exhibiting the numerical magnitudes of the two 

 principal forces and the angle made by one of them with y, for 

 121 points in case (1), and for 231 points in each of the other 

 cases. By means of these numbers, diagrams are formed, exhibiting 

 in each case the directions of the lines of compression-thrust and 

 tension-pull in every part of the beam. 



"Photochemical Researches. — Part. V. On the Measurement of 

 the Chemical Action of Direct and Diffuse Sunlight." By R. W. 

 Bunsen, For. M.R.S., and H. E. Roscoe, B.A. 



In one of the four communications which the authors have already 

 had the honour of presenting to the Royal Society on the subject 

 of the measurement of the chemical action of light, the attempt was 

 made to determine experimentally the laws regulating the distribu- 

 tion of the chemical action of the sunlight and diffuse daylight on 

 the surface of the earth when the sky is perfectly unclouded and 

 the atmosphere clear. The methods of measurement there employed 

 do not, unfortunately, apply to the much more usually occurring 

 case of cloudy skies and hazy atmosphere. The aim of the present 

 communication is to describe an entirely different mode of measuring 

 the chemical action effected at any point on the earth's surface by 

 the total sunlight and diffuse daylight, under the most widely vary- 

 ing conditions of situation, climate, and state of the atmosphere. 



In spite of the various futile attempts which have been made to 

 register and measure the chemical action of light by means of photo- 

 graphic tints, it still appeared possible in this way to attain the 

 desired end. No instruments founded on such a mode of measure- 

 ment can yield reliable results unless we know the conditions under 



