ICO ANNUAL OF SCIENTIFIC DISCOVERY. 



SUN SIGNALS FOR THE USE OF TRAVELLERS. 



If a piece of looking-glass be held in such a position that a person at a 

 distance can see some portion or other of the sun's disc reflected in it; it 

 assumes the appearance of an exceedingly brilliant star of solar light. At 

 a recent meeting of the Royal Geographical Society, Mr. Francis Galton, the 

 Africa^ traveller, described an optical arrangement he had devised, by which 

 the si-mailer may know whether he is holding the mirror aright. The 

 smallest size of hand heliostat can literally be carried in the waistcoat 

 pocket, yet, by its means, whenever the sun is shining, a signal can be 

 instantly made that shall be visible to the entire neighborhood of any given 

 spot within sight. A distance of twelve miles, on a day of average clearness, 

 is well within the power of this little instrument. If the flash be replied to, 

 a regular communication can be carried on, in which the signals are varied 

 by gentle movements of the hand that cause the flash to be seen and to 

 disappear alternately; words and sentences are communicated by a notation 

 of long and short flashes, identical with the notation of long and short beats 

 that is used in Morse's electric telegraph. 



ON THE SEPARATION OF THE HEAT AND LIGHT OF THE SUN'S RAYS 



IN THE EYE. 



A paper has recently been communicated to the French Academic des 

 Sciences, by J. M. Janssen, giving an account of a series of experiments un- 

 dertaken by him to ascertain how large a portion of the heat-rays pass 

 through the central portions of the eye and reach the retina at the back. 

 His experiments show that all the rays of heat are absorbed before they 

 reach the retina, two-thirds by the cornea, and the other third by the 

 aqueous humor. 



THE MECHANICAL THEORY OF HEAT. 



The following paper, communicated by Mr. D. Y. Clark, C. E., to the 

 London Engineer, sets forth in a popular manner what is now understood by 

 the so-called " mechanical theory of heat." The principle of this theory of 

 heat is, that, independently of the medium through which heat may be 

 developed into mechanical action, the same quantity of heat converted is 

 invariably resolved in the same total quantity of mechanical action. For 

 the exact expression of this relation, of course, units of measure are estab- 

 lished, in terms of the English foot, as the measure of space; the pound 

 avoirdupois, as the measure of weight, pressure, elasticity; and the degree 

 of Fahrenheit's scale, as the measure of temperature and heat. Work done 

 consists of the exertion of pressure through space, and the English unit of 

 work is the exertion of one pound of pressure through one foot, or the rais- 

 ing of one pound weight through a vertical height of one foot, briefly, a 

 foot-pound. The unit of heat is that which raises the temperature of one 

 pound of ordinary cold water by one degree Fahrenheit. If two pounds of 

 wafr In- rai-ed one degree, or one pound be raised two degrees in tempera- 

 tun-, the expenditure of heat is, equally in both cases, two units of heat. 

 Similarly, if one pound weight be raised through one foot, or two pounds 

 l>e raised through two feet, the power expended, or work done, is 

 v in both cases two units of work, or two foot-pounds. From these 

 definitions, then, the comparison lies between the unit of heat, on the one 



