August, 1887.] ELLIOTT SOCIETY. 173 



iu type, but not yet published, and sends me the manuscript of that portion 

 relating to the spread of P. rapcB in the southern states ; he also sends a copy 

 of the map, to accompany his paper, sho\ving the centres and areas of distri- 

 bution of P. 7Xipc6, from 1880 to 1886. These I lay on the table for the inspec- 

 tion of our members. Mr. S. regards Charleston as one of the centres of dis- 

 tribution, the insect not having come to the City from the surrounding country, 

 as I had supposed, but brought hither by some coasting vessel. 



A portable and easily made Heliotrope. 



BY PROF. LEWIS R. GIBBES. 



A Heliotrope is an instrument by which the course of the sun's rays may be 

 so changed b}^ the experimenter, as to be directed with certainty to a given 

 point at the distance of several miles. It may be made to subserve two im- 

 portant purposes; first, that of making known at the given point the position or 

 station of the experimenter, which it is necessary to do in a trigonometrical 

 survey ; secondly, to convey information from the experimenter to the given 

 point, by successive interruptions of the ray, when once accurately directed, 

 each interruption lasting but a short interval of time. Instead of interrupting 

 a fixed ray, the ray may be thrown upon the given point at intervals, forming 

 successive flashes of light, and in each of these methods an alphabet, corres- 

 .ponding to that of Morse, may be used, short and long flashes or interruptions 

 corresponding to the dot and dash of Morse. The art of thus communicating 

 a knowledge of the position of the experimenter, or of his thought, has been 

 called Heliography, and the instrment is sometimes called a Heliograph. 



Several forms of this instrument have been contrived by different authors. 

 One method, tolerably obvious and easy to comprehend is this: Place two 

 stands or supports, A and B, a few feet apart, in aline with the distant given 

 point, B being nearest to that point. Let each stand carry a screen one foot 

 square, each having a circular hole 3 or 4 inches in diameter cut in the middle 

 of it, that on B being a little smaller than the other ; the screens being so ar- 

 ranged that on looking through them from A to B, the given distant point may 

 appear to occupy the centres of each of the apertures A and B at thesame 

 time. Then if with a common hand mirror, a beam of sunlight be reflected 

 through A to B, the smaller, so as to form a circular ring of hght around the 

 edge of B and concentric with it, it will be evident that the rest of the beam 

 which passes thi-ough B, will be properly directed to the distant point. Such 

 a beam is easily visible in good weather 50 or 60 miles or even more. 



We can now obtain a second and more compact form of Heliotrope, by sup- 

 posing a pair of screens, similar to those above but smaller, to be attached to 

 the tube of a telescope, with the centres of theu' apertures carefully adjusted 

 to be in a line parallel to the optical axis of the telescope ; a small mii-ror, 

 movable about two axes at right angles to each other, being also attached to 

 the tabe of the telescope, near the eye-end to reflect the light through the 

 apertures. When the distant point is seen through the telescope in the centre 



