358 SCIENTIFIC AMUSEMENTS. 



known to act similarly, whilst occasionally a drop of dew or rain 

 on a leaf on a summer's day acts as a burning glass and scorches 

 the leaf. '. .' 



Expt. 385. To make a Burning Glass of Ice. Fill an evaporat- 

 ing basin, the shape of which is as near that of a hemisphere 

 as possible, with cold water, and then surround it with a mix- 

 ture of snow and salt (Expt. 21), renewed when requisite until 

 the water is frozen to a solid mass. Detach this from the basin 

 by turning it upside down and pouring a little warm water over it ; 

 as the heat penetrates through the porcelain or earthenware the ice 

 in contact with the basin thaws ; and by and by the mass of ice 

 drops out in the form of a rough piano convex lens. Place this in 

 the sun so that the beams fall directly upon either the flat or the 

 convex side ; the less irregularly shaped the rough ice lens is the 

 more nearly will both the visible light and the heat rays be con- 

 verged to a focus. If tolerably clear and accurate in form, the heat 

 rays will be found to pass readily through the ice, and will cause 

 the mercury to rise rapidly in a thermometer placed near the visible 

 focus ; with a lens some 8 or 10 inches in diameter and a mode- 

 rately hot sun, paper, &c., may readily be fired. 



A still better way of preparing the ice lens is to get a solid block 

 of clear ice with one flat surface (either naturally flat, or rendered 

 so by rubbing the block on a warm metal plate, &c.) ; with a hot 

 poker carefully applied melt away some of the ice on the other 

 side so as to make it roughly convex, and complete the shaping 

 process by inverting a hemispherical basin over the lump, pouring- 

 hot water over it from time to time, moving the basin about and 

 turning it round so as to thaw the upper surface of the ice block 

 down to the proper shape. 



Expt. 386. Sunshine Recorder. A useful application of the 

 principle of the burning glass is made in an instrument designed 

 to measure the amount of sunshine visible during the day. A 

 ball of glass or a hollow glass globe filled with water is set in the 

 sun, supported by a light frame, with a piece of stout white paper 

 so arranged that some part of the surface of the paper is in focus 

 at all hours of the day, the particular part depending on the hour, 

 i.e., on the position of the sun in the heavens. When the sun 

 shines its rays are focussed on the paper by the ball and a black 

 spot is burnt there ; if the sun continues shining, the focus alters 

 its place as the sun moves, and the spot becomes lengthened into 

 a black line. When the sun is hidden by clouds, &c., the burning 

 action is stopped ; so that at the end of the day the hours during 

 which the sun has shone brightly, as compared with the period 

 when it was under a cloud, can be readily measured by noticing 



