Angles of Minute Crystals. 309 



scale at which the index stands. The eye-piece must be a;!;ain 

 turned till that thread has passed over the whole angle, and co- 

 incides with the other side of it. The difference of position of 

 the index read off will give the measurement of the angle. 



The operation requires some neatness of manipulation, but 

 this is acquired by practice ; and if the operation be repeated 

 and a mean taken of the errors which occur each time, it will 

 be easy to come very near the truth. 



In this way the angles of crystals not more than the four or 

 five millionth of an inch in size may be accurately ascertained. 

 And the introduction of measurement in such cases has surel}* 

 a great advantage in this respect, that such crystals are more 

 perfect and sharp than almost any that can be obtained of a 

 large size ; and that frequently a whole family of crystals, in all 

 their correlative forms and varieties, will be found lying toge- 

 ther ; such as, in any size adequate for manipulation, the dili- 

 gent search of a whole lifetime could not have collected. 



Edinburgh, 8th April 1835. 



Description of a New Spring for Shutting a Door, xchicli goes 

 both ways. By Alexander Beattie, Foreman to Mr 

 RoBEKT Ritchie, Ironmonger to the King, 241. High 

 Street, Edinburgh.* With a Plate. 



The spring is contained in an iron-box, 11 inches by 7, and 

 1^ inches deep, and consists of two horizontal wheels, marked 

 on the section AA, Plate V., lying close above, and parallel 

 to, one another, moving on the same axis with the door, to which 

 the chains BB are attached, and moves with the wheels ; the 

 other ends of these chains are fixed to the levers CC, which move 

 on the pivots DD, close to which the triple springs, EE, are 

 made to press upon these levers. 



When the lever or door is pressed or opened to the rioht 

 hand, the under wheel is forced round, drawing back the lever 

 C by means of the chain B, which lever the spring E presses 



" Head to the Society of Arts for Scotland 11th December 1033. 



