256 ANNUAL OF SCIENTIFIC DISCOVERY. 



matical principles, and will be useful in high latitudes where the needle 

 traverses badly. Its accuracy depends on the correctness of the chronom- 

 eter, by which the index, or gnomon, is moved, and upon its adjustment to 

 the meridian of the place. 



ABBOT'S HOROMETER. 



The following is a description of a new nautical and astronomical in- 

 strument recently invented by the Rev. Amos Abbot, a missionary con- 

 nected with the American Board : 



A plane metallic hemisphere of ten-inch radius, with a graduated arc and 

 an orthographic projection of lines of latitude divided by dots into minutes 

 of time, and numbered from six o'clock towards the arc for the A. M., and 

 from the arc for the p. M.. is the foundation. Moving from the centre of this 



' O 



projection is an index arm, like a quadrant, with a Vernier, reading to half 

 minutes, and upon this arm, sliding in a groove, and at right angles to it is 

 a bar, graduated for a scale of altitudes and comprehending the appropriate 

 corrections. This scale-bar, of course, moves with the index arm, and is 

 always perpendicular to it, and across it a plane glass, with fine lines upon 

 its surface, is made to slide so that it may be set to any given altitude. By 

 this simple combination of parts, the time from an altitude of the sun, moon, 

 planet, or star, is readily worked. Latitude, by various means, is deter- 

 mined ; a lunar distance is cleared ; azimuth, without a compass, is found ; 

 and, in short, all spherical problems arc solved by inspection. The plan of 

 the instrument is obvious to a person familiar with spherical trigonometry, 

 correct, and the execution of it so nice, that its accuracy is easily demon- 

 strated by examples. 



CAVEXDY'S NAUTICAL TRIPOD. 



An instrument for obtaining nautical observations in thick weather, in- 

 vented by Capt. Cavendy, of New York, consists of a metallic tube, sup- 

 ported by a tripod on a universal hinge, so as to keep it in a vertical posi- 

 tion, with its point constantly to the zenith. Through this tube the position 

 of the sun is ascertained at meridian, and by the angle obtained between it 

 and the zenith the basis for calculating correct observations is obtained, 

 while the use of the quadrant to give correct observations would require a 

 clear horizon. This instrument is highly commended. 



CONTRIBUTIONS TO OUR KNOWLEDGE OF THE CONDITIONS OF 



THUNDER STORMS. 



Mr. John AVis", the eminent American JEronaut, has recently published, in 

 the New York Tribune, the following very curious observations on the 

 physical aspect of thunder storms, which have been made by him from, time 

 to time during his numerous balloon ascensions : 



A Slorm viewed from above the clouds has the appearance of ebullition. The 

 upper surface of the cloud is bulged upward and outward, and has the re- 

 semblance of a vast sea of snow boiling and upheaving from internal con- 



