
\begin{figure}
\centering
\begin{tikzpicture}
  \fill[semithick,blue] (0,0)  circle (0.25) (1,0) circle(0.25)  (2,0) circle(0.25)   (3,0) circle(0.25) 
  (4,0) circle(0.25) (5,0) circle(0.25) (6,0) circle(0.25) (7,0) circle(0.25); 
  \fill [semithick,blue] (0,-2)  circle (0.25) (1,-2) circle(0.25)  (2,-2) circle(0.25)   (3,-2) circle(0.25);  
  \draw[thick,->] (0,-0.25)--(0,-1.75);
  \draw[thick,->] (4,-0.25)--(0,-1.75);  
  \draw[thick,->] (1,-0.25)--(1,-1.75);
  \draw[thick,->] (5,-0.25)--(1,-1.75);
  \draw[thick,->] (2,-0.25)--(2,-1.75);
  \draw[thick,->] (6,-0.25)--(2,-1.75);  
  \draw[thick,->] (3,-0.25)--(3,-1.75);
  \draw[thick,->] (7,-0.25)--(3,-1.75);
  
  \fill [semithick,blue] (0,-4) circle (0.25) (1,-4) circle(0.25); 
  \draw[->] (2,-0.25)--(2,-1.75); 
  \draw[thick,->] (0,-2.25)--(0,-3.75);
  \draw[thick,->] (2,-2.25)--(0,-3.75);  
  \draw[thick,->] (1,-2.25)--(1,-3.75);
  \draw[thick,->] (3,-2.25)--(1,-3.75);
  
  \fill [semithick,blue] (0,-6) circle (0.25);
  \draw[thick,->] (0,-4.25)--(0,-5.75);
  \draw[thick,->] (1,-4.25)--(0,-5.75);
\end{tikzpicture}
\end{figure}


