OF THE EXPEDITION. XXI 



and circle, will cut off radiant heat ; and the instrument should he mounted on stone piers. The 

 eye-end should be fitted with two micrometer microscopes, the plate or diaphragm of that which 

 moves vertically to carry not less than five equidistant horizontal wires. As this construction 

 will permit a trough of mercury to remain always supported between the piers, the permanence 

 of the nadir point may be tested after every observation without much loss of time, or the annoy- 

 ance and unsteadiness when mounted on a tall ladder. Moreover, the observer remains con- 

 stantly in the same natural and unrestrained position most conducive to accurate observations, 

 and never risks disturbing his circle when rising. A reversing carriage and the basin of mer- 

 cury will obviate any actual necessity for a level, and every eye-piece should be so constructed 

 that it may be used in collimating.] 



"I am not aware of any suitably- constructed circle unemployed which could be obtained for 

 the Expedition, and consequently application was made to Messrs. Pistor & Martins, through 

 the United States legation at Berlin, for an estimate of the time that will be required and the 

 probable cost of such an instrument. In reply, these artists offer to complete one within nine 

 months after the receipt of an order, for about $2,100. The fidelity with which they executed 

 the transit instrument mounted in the prime-vertical of the Washington observatory, as well as 

 their punctuality (to a day) in fulfilling their contract for its delivery, added to the character 

 which the Pistor circle at the Berlin observatory has obtained for the senior partner, would 

 induce me to give the order to these gentlemen, and leave all details to their known judgment 

 and skill. 



"Second. An achromatic telescope of about five feet focal length, and aperture of forty-eight 

 lines, or thereabout. The instrument in view was purchased for the United States exploring 

 expedition, and is mounted on a tripod stand. It will require alterations and additions to its 

 mounting and equipment, as it is only furnished with annular and reticulated micrometers. I 

 have no doubt that Mr. Young, of Philadelphia, can mount it as an equatorial satisfactorily, 

 and would recommend that it be done in the simplest style, viz : that of the comet-searchers 

 made at Munich by Mr. Merz. It will probably be necessary to send abroad for a filar-microm- 

 eter ; and to facilitate differential measures with objects in distant parts of the field, it is 

 intended that its movable diaphragm shall contain not less than five equidistant wires. A 

 similar number of transverse wires will permit the instrument to be used as a transit in differ- 

 ences of right ascension. 



"Third. A sidereal clock and three chronometers. The clock wanted was also purchased for 

 the exploring expedition, and remains in charge of Commander Wilkes. Should he still have 

 it in use, one must be purchased. One of the chronometers should be regulated to sidereal 

 time, and another be of ' pocket' form. 



"Fourth. A sextant and an artificial horizon. 



"Fifth. A declinometer. The instrument referred to was used in the observations made on 

 Capitol hill from 1840 to 1842, and is described in the volume of magnetical and meteorological 

 observations published in 1845 by order of the Senate U. S. It will require slight modifica- 

 tions to render it more effective, which may be made by any ordinarily good mechanician. 



"Sixth. A Fox's deflector. There is one of these instruments, not in use, at the Washington 

 observatory. Prof. Bache informs me that it is seriously defective in its pivots or jewelled 

 holes, or both, and requires thorough examination and repair. This, I have ascertained, may 

 be done properly in New York. 



"Seventh. A standard and two mountain barometers. The former should be of Newman's or 

 I'istor's construction, and all of them carefully compared with the standard of the Washington 

 observatory. 



"Eighth. Thermometers. There will bo required two standards ; six common instruments, 

 divided to single degrees 'from 10 to + 120; two pairs of Kutherford's self-registering; 

 two terrestrial and two solar radiating, and two wet-bulb. The scales of the last should be 



