84 
THE YOUNG SCIENTIST. 
Mechanical Equivalent of Heat.— It does 
not appear to be ^4-eueraliy known that the value 
of the mechanical equivalent of heat has within a 
few years been corrected. It is generally referred 
to by mechanical writers at 772 foot pounds. Dr. 
Joule repeated his famous experiments in 1876, 
nearly six years ago, with extraordinary precau- 
tions, and the mean result of sixty experiments 
gave 774.1 foot pounds with a possible error of 
1.400, on account of the " thermometric scale 
error." This value should be used in all calcula- 
tions relating to the value of heat as a motive 
power. 
> • ■ 
EXCHANGES. 
Only those who are subscribers, and whose names 
are entered on our books have the privilege of inserting ex- 
changes. 
Exchanges must be on separate slips of paper or postal 
cards. If mixed with business matter in letters or cards they 
are filed away and never reach the printer. 
Exchanges must not exceed thirty words. 
Buying and selling belong to the advertising department. 
We reserve the right to omit the exchange column, when 
we have not room for it, and the amount of space at our dis- 
posal will regulate the number of insertions given to each 
exchange, the preference being always given to those v/ho 
have not previously used our columns. 
Blow-pipe set (cost $io), large illustrated family Bible 
(cost $7.50), for Wood's Botany, Dana's Geology, or 
printing press and type. H. W. Noble, Box 134, South 
Dansville, N. Y. 
Wanted, offers for fossil shells from the west coast of 
Africa. A. W. Seward, 187 E. 71st St. 
I have a number of birds' eggs I should like to trade for 
other birds' eggs; send for list of trading eggs. A. G. G., 
Box 26, Summit, New Jersey. 
For a microscope stand or offers, a 36-inch turning lathe 
(new) and a number of scientific books; send for list. A. 
Kendall, Somerville, Mass. 
Wanted services of amateur printer with small office (or 
large), in exchange for an interest in a well-estabhshed 
weekly paper, good size. Eagle Office, Plymouth, N. H. 
American Agriculturist microscope, new, cost $15, for 
amateur photographic apparatus, lever watch, 22 calibre 
breech-loading sporting rifle or magic lantern of equal value. 
Wm. A. Walker, Rockland, R. I. 
I. T. Bell, Franklin, Pa., has back numbers of Scientific 
American, Forest and Stream, and Young Scientist, good 
flute, chessmen, books, etc., for good magic lantern, revolver, 
scarce coins, or offers. 
A steel spring bracket-saw, 2 doz. saw blades, and about 
I doz. patterns for a good stylographic pen or offers; send 
postal before exchanging. Geo. Oakley, Rutherford, N. J. 
A rosewood banjo, 11 inch head, 12 ntits, fully strung, 
together with lot of dime banjo music and instructions, for 
microscope, Webster's Unabridged, monkey, squirrel and 
cage, or offers. J. DeWitt Clark, P. O. Box 37, Brooklyn, 
N. Y. 
For self-inking printing press and type, one Henry rifle, 
16 shot, 44 cal., cost $40, in good order, or one double barrel 
shot gun, barrels London fine twist, or offers. J. B. Garri- 
son, Belton, Bell Co., Texas. 
A new 40 diameter microscope, corals, and minerals, for a 
stylographic pen, air gun, or offers J. McBride, 54 Garley 
St., Chicago. 
What offers for "The Chefs D'Oeuvre D'Art, Paris Ex- 
hibition 1878," cost $16, good as new, bound; i large 
photo camera, complete, cost $95 ; Scientific American from 
1859 to 1880, 4 vols, bound. D. Keesler, Stony Point, N. Y. 
Sea shells of Atlantic coast, scientifically marked, for 
books, old or new magazmes, Indian relics, natural history 
specimens, apparatus; anything Wilfred Leon Miller, 
P. O. Box 392, Cape May, N. J. 
A dulcimer that cost $20, books and fossils, for a tele- 
scope, microscope, fossils, Indian relics, or offers. E. J. 
Votaw, Salem, Iowa. 
A self-inking model press, 5 x 71^, cost $23 ; $90 worth of 
scroll-saw designs in lots to suit; Scientific American, 1880; 
Palliser's Useful Details, $3; for photo outfit or offers. 
C. H. Parker, Coldbrook Springs, Mass. 
Telegraph key and sounder, Frank Leslie's Illustrated 
Magazines, mounted microscopic objects, natural curiosities, 
for bull's-eye condenser, photograph camera, Unabridged- 
Webster Dictionary, or offers. W. T. Alan, Greenville 
Mercer Co., Pa. ' 
Large magic lantern, spectroscope, pantagraph, for en- 
larging drawings, rosewood writing desk, man's saddle, for 
microscope, photo outfit or offers. Thomas Walters, Bergen 
St., East of Brooklyn Ave., Brooklyn, N. Y. 
Model card press, chase 3%^ x 514^, with 4 fonts type, cases, 
etc., for good foot lathe. Mills Day, 2 Farmington Av.i 
Hartford, Ct. 
French microscope, inclines to any angle, delicate fine 
motion, neat upright case, cost $17.50, for scientific books; 
send list to W. Fitz, P.O. Box 2852, New York. 
Wanted, Quinby's New Bee Keeping, for Our Own Birds 
of the United States, by W. S. Baily ; new. Joseph 
Anthony, Jr., Coleta, Whiteside Co., Illinois. 
Printing type, small pica ; large and small capitals, and 
small letters; 60 to 65 lbs., including two large cases; also 
gothic nonpariel, for trade. J. Siler, 1242 Broadway, St. 
Louis, Mo. 
One years' copy of Aldine, cost $5.50, and German 
accordion, with instructions how to use it, cost $5.00, for 
offers. Chas. Dempwolf, 86 Benson St., Paterson, N.J. 
A Novelty printing press, with type, prints a form 5x7 
inches, cost $40.00, for a microscope, photographic appa- 
ratus, or offers. T. W. Patterson, Warsaw, N. Y. 
$80 worth of (nearly new) printing material, minerals, 
fossils, Aboriginal relics, etc., for scientific books, good 
American watch, and offers. F. M. Farrell, Cobden, Union 
Co., Ills. 
Rocks, minerals, fossils and fresh water shells for micro- 
scope stand, $20 to $75, also for objectives, turning lathe 
type, and tools. D. D. Babcock, South Dansville, N. Y. 
Mechanical Electricity, Medical Chemistry, by Robert 
Hare; Fruits and Farinacea, by Trail; Complete Herbalist, 
by O. P. Brown ; for mounted objects for the microscope or 
offers. J. B. Playter, Bristow, Butler Co., Iowa. 
Violin (cost $10) and 5-shot 32-cal. revolver (cost $4), for 
photo-camera lens and tube, watch, parlor rifle, extra 
well-bred canary birds, or offers. C. Maides, St. Louis, Mo. 
To exchange for a wood-turning lathe or offers, a Young 
America Self-inking Printing Press, with type and outfit 
complete ; chase 3x5 inches. Embury McLean, 318 Bloom- 
field St., Hoboken, N. J. 
What offers for " Our First Century," cost $7.00, bound 
in sheep, over 400 illustrations, 1,000 pages; also small 
Ruby Magic Lantern, cost $1.50. Box 217, So. Manchester, 
Conn. 
Magic lantern, in good order, condensing' lens, 2^^ in. 
diameter, 8 slides. For small photographic camera or offers. 
H. A. Giddings, 4 Union Place, Classon Ave., Brooklyn. 
Printing press, type, etc., worth $13, for offers. E. G. 
Vogeley, 1010 Bradford St., Pittsburgh, Pa. 
Fifty canary birds, with and without crests; fancy 
poultry, conch shells, strombus alatus. For small printing 
press, books on natural history, or offers. Frank Lattin, 
Gaines, N. Y. _ 
A good plain 7 shot revolver, 22 calibre, for a Students' 
inch or half-inch objective. Frank F. Colwell, Urbana, 
Ohio. 
* • ■ 
Grood Writing:.— No one can write rapidly and 
legibly without a good pen, and unfortunately 
there are many pens in market that are not 
good. The majority of writers find that Ester- 
brook's Falcon steel pen suits their hand, and 
our experience is that no pen retains its good 
qualities longer. 
