One of the most perplexing problems of cosmogony is the great speed of the spiral nebulae. Their radial velocities average about $600$~km.\ per~sec.\ and there is a great preponderance of velocities of recession from the solar system. It is usually supposed that these are the most remote objects known (though this view is opposed by some authorities), so that here if anywhere we might look for effects due to a general curvature of the world. De~Sitter's theory gives a double explanation of this motion of recession; first, there is the general tendency to scatter according to equation~\Eq{(70.22)}; second, there is the general displacement of spectral lines to the red in distant objects due to \index{Displacement of spectral lines to red, in sun!in nebulae}% the slowing down of atomic vibrations~\Eq{(67.4)} which would be erroneously interpreted as a motion of recession. \PageSep{162} The most extensive measurements of radial velocities of spiral nebulae \index{Nebulae, velocities of}% \index{Spiral nebulae, velocities of}% have been made by Prof.\ V.~M. Slipher at the Lowell Observatory. He has kindly prepared for me the following \hyperref[table:70.1]{table}, containing many unpublished results. It is believed to be complete up to date (Feb.~1922). For the nebulae marked~(*) the results have been closely confirmed at other observatories; those marked~(\dag) are not so accurate as the others. The number in the first column refers to the ``New General Catalogue,'' \Title{Memoirs R.A.S.}, vol.~49. One additional nebula \NGC\ 1700 has been observed by Pease, who found a large receding velocity but gave no numerical estimate. \begin{table}[hb] \centering \textsc{Radial Velocities of Spiral Nebulae} \\ $+$ indicates receding, $-$ approaching \phantomsection\label{table:70.1}% \[ \footnotesize \begin{array}{@{}r@{\qquad}r@{\ }r@{\quad\ }r@{\ }rc|r@{\qquad}r@{\ }r@{\quad\ }r@{\ }rc@{}} \multicolumn{1}{c}{\text{\NGC}} & \multicolumn{2}{c}{\RA} & \multicolumn{2}{c}{\text{\quad Dec.}} & \multicolumn{1}{c|}{\text{Rad.\ Vel.}} & \multicolumn{1}{c}{\text{\NGC}} & \multicolumn{2}{c}{\RA} & \multicolumn{2}{c}{\text{\quad Dec.}} & \multicolumn{1}{c}{\text{Rad.\ Vel.}} \\ & h & m & ° & ' & \multicolumn{1}{c|}{\text{km.\ per sec.}} & & h & m & ° & ' & \multicolumn{1}{c}{\text{km.\ per sec.}} \\ 221 & 0 & 38 & +40 & 26 & -\9 300 & 4151\rlap{*} & 12 & 6 & +39 & 51 & +\9 980 \\ 224\rlap{*} & 0 & 38 & +40 & 50 & -\9 300 & 4214 & 12 & 12 & +36 & 46 & +\9 300 \\