VII. RECORDING INSTRUMENTS. 707 



any point on which may be found with reference to a table prepared for the 

 purpose. The drum revolves freely on the stand (c). The paper to bo 

 examined is placed over the graduated strip, being held in position by the 

 two clamps attached to the stand. 



By means of the semi-circular hole, which has been punched into the 

 exposed paper, the tint of the paper and of the graduated strip are brought 

 side by side, and by revolving the drum with the hand various portions of 

 the graduated strip are brought successively into juxtaposition with the tint 

 of the exposed paper, and when the two tints ai-e seen to be identical; the 

 value of the exposed tint is known, corresponding of course to the one for 

 that point on the graduated strip, and ascertained by reference to the table 

 of intensities. A number of such readings for each tint are obtained and 

 the mean taken as correct. These comparisons must be made by the light 

 of the monochromatic soda flame. 



Beads of sodium carbonate (//) on the platinum wires (e, e,) are held by 

 means of the stands (/,./j) in the flame of the Bunsen burner (#), and the 

 rays of light are concentrated on to the strip by the lens (A). When gas is 

 not available the spirit lamp () is employed. 



It now remains alone to divide the value of the tint thus obtained by the 

 number of seconds which the tint has been exposed, to determine the in- 

 tensity of the light for one second of time, Avhich is the standard adopted. 

 Two or three of the discs obtained each hour are thus read off, and the mean 

 taken as representing the correct intensity. 



2877a. Insulator and Whirling Apparatus. 



Capt. Abney, E.E., F.R.ti. 



This instrument is based on the same principles as that of Professor Roscoe, 

 but it was constructed with the idea that its portability may cause it to be 

 employed in localities where his larger apparatus might be too bulky. 



It consists of a revolving cylinder (round which is wound sensitive 

 paper), driven by clockwork enclosed in a cover. In the cover is a narrow 

 slit, extending across the cylinder, over which is pressed a black glass 

 wedge of known graduation. The light falls through the wedge on to the 

 paper in different gradations, according to the intensity of the light. The 

 integration of the darking is obtained by placing the cylinder in a whirling 

 machine, and the position of the standard tint is noted on a scale which 

 depends on the graduation of the wedge. The total insolation is thus arrived 

 at. When Professor Roscoe's and this instrument are worked together the 

 results are accordant. 



2879a. Pull sized Lever Anemometer for registering the 



pressure of the wind. C. O. f\ Cator, M.A, 



The pressure-plate is circular, has an area equal to 1 square foot, and is 

 kept constantly face to wind by a vane (or by windmill fans) ; as it is driven 

 forward by the wind, its motion is conveyed by means of a wire to a pencil, 

 which continuously records every movement on paper revolving by clockwork 

 below. 



The peculiarity of this instrument is, that, instead of employing springs as 

 the resisting medium, which from their proximity to the pressure plate and 

 consequent continual exposure to the weather, cannot always preserve the 

 same strength and elasticity, the resistance is furnished by a system of leverage. 

 This consists of two eccentric curves of different sizes rigidly connected 

 together and revolving on the same axle, of which the curvatures decrease- 

 respectively in opposite directions, so that the effect is doubled. Round the 

 larger one a cord is carried, from which hangs a fixed weight. Round the 



