TRANSACTIONS OF SECTION A. 461 



stations exist whence mountain tarns can he seen Ij'ing at a much lower level 

 than this, and where even the higliest cirrus could be measured with satisfactory 

 precision. 



Useful regular work might he done by a meteorologist whose station was at a 

 height of even 50 feet above a pool, supposing it to be so well sheltered from the 

 wind as to frequently afford perfectly good reflexions with, say, 1' maximum error. 

 Very shallow water is much stiller than deep water, as waves cannot be propagated 

 over it ; thus we may often see wonderfully good reflexions in road-side splashes and 

 puddles, in the intervals between puffs of wind. The most stagnant air is in the 

 middle of a high and broad plantation, where there is also plenty of dense under- 

 wood. Detached puddles of water in broad ruts would be a good equivalent for 

 a pool. As regards the size of the pool, if we let fall a perpendicular k from the 

 mercury trough to the level of the water, the utmost portion of the surface of the 

 pool that canlje used with effect extends between the distances of about hk and 

 4k from the base of the perpendicular. The angles of depression would be then from 

 G4° to 14° about, or say, a range of 50°. The usual limits would be from k to 3^-, 

 or between 45° and 18°, being a range of 27°. 



Improved Heliograph or Sun Signal. By Tempest Andersox, M.D., B.S.c 



The author claims to have contrived a heliograph, or sun-telegraph, by which 

 the rays of the sun can be directed on any given point with greater ease and 

 certainty than by those at present in use. 



When the sun's rays are .reflected at a small plane surface considered as a point, 

 the reflected rays form a cone, whose vertex is at the reflector and whose vertical 

 ano-le is equal to that subtended by the sun. Adding to the size of the mirror 

 adds other cones of light, whose bounding rays are parallel with those proceeding 

 from other points of the mirror, and only distant from them the same distance as 

 the points on the mirror from which they are reflected. Hence increasing the size 

 of the mirror only adds to the field to which the sun's rays are reflected a diameter 

 equal to the diameter of the mirror, and this at any distance at which the sun- 

 signal would be used is quite inappreciable. Adding to the size of the mirror adds 

 to the number of rays sent to each point, and hence to the brightness of the visible 

 flash, but not to the area over which it is visible. _ 



By the author's plan, an ordinary field-glass is used to find the position of the 

 object to be signalled to, and to it is attached, in the position of the ordinary sun- 

 shade, a small and light apparatus, so arranged that when the mirror is turned to 

 direct the cone of rays to any object within the field of view of the glass, an image 

 of the sun appears in the field, at the same time as the image of the distant object, 

 and magnified to the same degree, and the part of the field covered by this image 

 is exactly that part to which the rays are reflected, and at which some part of the 

 sun's disc is visible in the mirror. 



A perfectly plane silvered mirror, A, takes up the rays of the sun, and when in 

 proper position reflects them parallel with the axis of D, which is one barrel of an 

 ordinary field-glass. The greater part of the liglit passes away to the distant 

 object, but some is taken up by the small silvered mirror E, which is placed at an 

 ano-le of 45° to the axis of D, and reflected at a right angle through the unsilvered 

 plane mirror, F, and the convex lens, K, by which it is brought to a focus on the 

 white screen, H, which is placed in the principal focus of K. The rays from this 

 ima^e diverge in all directions, and some are taken up by the lens K and restored 

 to parallelism ; some of these are reflected by the unsilvered mirror, F, down to the 

 field-o-lass, D, and if this is focussed for parallel rays, as is the case in looking at 

 distant objects, an image of the sun is seen projected on the same field of view as 

 that of the distant object. As the mirrors E and F are adjusted strictly parallel, 

 the rays proceeding from F into the field-glass are parallel and in the opposite 

 direction to those going from the mirror A to E, which form part of the same 

 pencil as those going to the distant object. Hence the image of the sun seen in 

 the field exactly covers the object to which the sun-flash is visible, and in whatever 



