426 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



during every twenty-four hours. Of course it would be a great desidera- 

 tum to have the record of changes continuous. This can only be accom- 

 plished by self-registering instruments. Many attempts have been made 

 to connect recording attachments with the barometer and thermometer. 

 Thus far they have failed, because the power required to move the index 

 being derived from the metal of the instrument, necessarily retards its 

 changes; but the advance in one branch of art often helps another. This 

 has been the case with photography, which has happily presented tlio 

 means for the most perfect registering of meteorological changes, without 

 in the least affecting the instruments used. The invention is a simplo 

 combination of photographic paper, with apparatus before iu use; yet it 

 must be regarded as the most perfect philosophical instrument ever made. 

 A description of its application to one of the most delicate of instruments — 

 the dipping needle — will illustrate its importance. To the end of the 

 needle is attached a small mirror, and to the other a counter weight, so 

 that the needle will still be perfectly balanced. At a short distance, 

 obliquely from the needle, and having no connection with it, is placed a 

 cylinder, which turns on a vertical axis, and by clock-work machinery 

 makes just one revolution in twenty-four hours. Around this cylinder is 

 attached a sheet of paper, made sensitive to the action of light, and around 

 the whole is a stationary case, having an opening* through its whole length 

 on the side towards the needle. At an equal distance, in an oblique direc- 

 tion on the other side, is placed a lamp, the rays of which strike the little 

 mirror and are deflected upon the photographic paper, making a continuous 

 mark, ranging in altitude as the end of the needle moves upward or down- 

 ward. The graduated scale by which this altitude is measured should have 

 gradually' decreasing intervals from a central point, to compensate for the 

 curved motion of the mirror. At the end of every twenty- four hours another 

 sheet of paper is substituted. Thus the subtle magnetic force silently 

 records its own changes, with a constancy and correctness which human 

 hands can never equal. This photographic arrangement can be applied 

 to every philosophical instrument which measures by motion. Indeed, all 

 the measurements could be made side by side upon the same sheet, and 

 three hundred and sixty-five of them following in daily consecutive course 

 would form an Annual of Observations. The cost of this apparatus will 

 for some time prevent its general use; but by-and-by, as the public appre- 

 ciate the importance of watching, without intermission, tl>e motions and 

 forces of the material world, they will be sought after with avidity. And, 

 finally, the time will come when many a hill-top in every civilized country 

 will be crowned with an observatory, and filled with philosophical instru- 

 ments, which will be the favorite resort of those votaries of science who 

 have learned that the Creator communicates the immutable mandates con- 

 cerning matter only to the patient seeker. 



The subject of Mathematical Instruments was continued for the next 

 evening. 



